If someone were to come to us and said that he or she would like to become a Muslim, what would we ask him or her to do? The answer lies in the Holy Qur'an. The Qur'an is replete with verses directing the believers to believe in Allah, His Messengers, books revealed to all prophets, the angels, the Last Day, enjoining what is right and forbidding what is wrong, etc:

The messenger believes in what has been revealed to him from his Lord, and (so do) the believers; they all believe in Allah and His angels and His books and His messengers; We make no difference between any of His messengers (Qur'an 2:285)

They believe in Allah and the last day, and they enjoin what is right and forbid the wrong and they strive with one another in hastening to good deeds, and those are among the good (Qur'an 3:114)

O you who believe! believe in Allah and His Messenger and the Book which He has revealed to His Messenger and the Book which He revealed before; and whoever disbelieves in Allah and His angels and His messengers and the last day, he indeed strays off into a remote error (Qur'an 4:136)

And there are many similar verses in the Qur'an. The articles of faith can be summarized into three:

1. Oneness of Allah
2. Prophethood
3. Day of Judgment

Oneness of Allah:

Oneness of Allah or Unity of Allah means that He is the True Creator of this universe, none shares in His attributes. He is the absolute Owner, none can repel what He does. He is the All Powerful, the Living, the All Knowing, etc. He possesses all the attributes of perfection. In other words, when we say that He is Knowledgeable, we mean that not only He knows everything but also it is not possible for Him to be ignorant of anything as ignorance is an attribute of imperfection.

And Allah's are the best names, therefore call on Him thereby, and leave alone those who violate the sanctity of His names; they shall be recompensed for what they did (Qur'an 7:180)

It is not allowable that any creature can do any act independent of Allah. Associating Allah with anyone in any way is one of the greatest sins which Allah does not forgive without true repentance:

Surely Allah does not forgive that anything should be associated with Him, and forgives what is besides that to whomsoever He pleases; and whoever associates anything with Allah, he devises indeed a great sin. (Qur'an 4:48)

Of course those who realize their mistake of associating others with Allah and sincerely repents, they will surely find Allah to be Forgiving as He forgives all sins and surely 'all sins' include association (Shirk) too:

Say: O my servants! who have acted extravagantly against their own souls, do not despair of the mercy of Allah; surely Allah forgives the faults altogether; surely He is the Forgiving the Merciful (Qur'an 39:53)

Allah has denounced associating Allah with others in the strongest terms throughout the Qur'an:

Surely whoever associates (others) with Allah, then Allah has forbidden to him the garden, and his abode is the fire; and there shall be no helpers for the unjust. (Qur'an 5:72)

O followers of the Book! do not exceed the limits in your religion, and do not speak (lies) against Allah, but (speak) the truth; the Messiah, Isa son of Marium is only a messenger of Allah and His Word which He communicated to Marium and a spirit from Him; believe therefore in Allah and His messengers, and say not, Three. Desist, it is better for you; Allah is only one Allah; far be It from His glory that He should have a son, whatever is in the heavens and whatever is in the earth is His, and Allah is sufficient for a Protector (Qur'an 4:171)

Say: My Lord has only prohibited indecencies, those of them that are apparent as well as those that are concealed, and sin and rebellion without justice, and that you associate with Allah that for which He has not sent down any authority, and that you say against Allah what you do not know (Qur'an 7:33)

They have taken their doctors of law and their monks for lords besides Allah, and (also) the Messiah son of Marium and they were enjoined that they should serve one Allah only, there is no god but He; far from His glory be what they set up (with Him) (Qur'an 9:31)

All the prophets' teachings started with the Unity of Allah:

Certainly We sent Nuh to his people, so he said: O my people! serve Allah, you have no god other than Him; surely I fear for you the chastisement of a grievous day (Qur'an 7:59)

To the 'Ad people, (We sent) Hud, one of their (own) brethren: He said: O my people! worship Allah! ye have no other god but Him will ye not fear (Allah)?"  (Qur'an 7:65)

And to Samood (We sent) their brother Salih. He said: O my people! serve Allah, you have no god other than Him (Qur'an 7:73)

And to Madyan (We sent) their brother Shu'aib. He said: O my people! serve Allah, you have no god other than Him (Qur'an 7:85)

Say: O people! if you are in doubt as to my religion, then (know that) I do not serve those whom you serve besides Allah but I do serve Allah, Who will cause you to die, and I am commanded that I should be of the believers (Qur'an 10:104)

Oneness of Allah is the cornerstone of all Islamic beliefs. In other words, all other beliefs are derived from it.

Prophethood:

Prophethood means that we believe in all the prophets Allah sent throughout human history starting with Adam (A.S) and ending with Muhammad (SAW). Islam was perfected through Prophet Muhammad (SAW) and the Holy Qur'an, the Final revelation has been preserved 100% by Allah. Hence, there is no need of any prophet after Muhammad (SAW):

Muhammad is not the father of any of your men, but he is the Messenger of Allah and the Last of the prophets; and Allah is cognizant of all things (Qur'an 33:40)

So far as being prophets of Allah and sent by Allah with true guidance, we do not make any distinction between any of the prophets. Denying any of the prophets amounts to denying all of them:

Say: We believe in Allah and (in) that which had been revealed to us, and (in) that which was revealed to Ibrahim and Ismail and Ishaq and Yaqoub and the tribes, and (in) that which was given to Musa and Isa, and (in) that which was given to the prophets from their Lord, we do not make any distinction between any of them, and to Him do we submit (Qur'an 2:136, 3:84)

However, Allah Himself has exalted some of the prophets over others in ranks:

We have made some of these messengers to excel the others among them are they to whom Allah spoke, and some of them He exalted by (many degrees of) rank; and We gave clear miracles to Isa son of Marium, and strengthened him with the holy spirit (Qur'an 2:253)

And your Lord best knows those who are in the heavens and the earth; and certainly We have made some of the prophets to excel others, and to Dawood We gave a scripture (Qur'an 17:55)

Glory be to Him Who made His servant to go on a night from the Sacred Mosque to the remote mosque of which We have blessed the precincts, so that We may show to him some of Our signs; surely He is the Hearing, the Seeing (Qur'an 17:1)

We believe that all the prophets faithfully received the divine Message and conveyed it to their people without any form of addition or subtraction. They were the best of people. They invariably followed what was revealed to them. They were truly guided by Allah without any misguidance in them:

And We gave to him Ishaq and Yaqoub; each did We guide, and Nuh did We guide before, and of his descendants, Dawood and Sulaiman and Ayub and Yusuf and Haroun; and thus do We reward those who do good (to others) (Qur'an 6:84)

And Ismail and Al-Yasha and Yunus and Lut; and every one We made to excel (in) the worlds:  and from among their fathers and their descendants and their brethren, and We chose them and guided them into the right way (Qur'an 6:86-87)

These are they on whom Allah bestowed favors, from among the prophets of the seed of Adam, and of those whom We carried with Nuh, and of the seed of Ibrahim and Israel, and of those whom We guided and chose; when the communications of the Beneficent Allah were recited to them, they fell down making obeisance and weeping (Qur'an 19:58)

Whomsoever Allah guides, he is the rightly guided one (Qur'an 18:17)

Hence, all the honored prophets of Allah (peace be upon all of them) were free from any kind of misguidance and therefore they were sinless (Ma'sum). May Allah send His blessings to the best of the Prophets, Muhammad (SAW) and his Family, and on all the prophets.

Day of Judgment:

Belief in the Last Day means that Allah is going to resurrect everyone one day and judge all men as to what they had done in this worldly life. He is going to reward those who believed in Him and obeyed Him sincerely, and punish those who disobeyed him. Those who will have mixed deeds, that is - good deeds mixed bad ones - it is up to Allah either to forgive them or punish them up to a certain time. We also believe that Allah has promised reward for those who do good deeds and He will keep His promise:

Nor do they spend anything that may be spent, small or great, nor do they traverse a valley, but it is written down to their credit, that Allah may reward them with the best of what they have done (Qur'an 9:121)

Men whom neither merchandise nor selling diverts from the remembrance of Allah and the keeping up of prayer and the giving of poor-rate; they fear a day in which the hearts and eyes shall turn about;  that Allah may give them the best reward of what they have done, and give them more out of His grace; and Allah gives sustenance to whom He pleases without measure (Qur'an 24:37-38)

And (as for) those who believe and do good, We will most certainly do away with their evil deeds and We will most certainly reward them the best of what they did (Qur'an 29:7)

Surely they who recite the Book of Allah and keep up prayer and spend out of what We have given them secretly and openly, hope for a gain which will not perish.  That He may pay them back fully their rewards and give them more out of His grace: surely He is Forgiving, Multiplier of rewards. (Qur'an 35:29-30)

And for all are degrees according to what they did, and that He may pay them back fully their deeds and they shall not be wronged (Qur'an 46:19)

And as to those who believe and do good deeds, He will pay them fully their rewards; and Allah does not love the unjust (Qur'an 3:57)

Then as for those who believe and do good, He will pay them fully their rewards and give them more out of His grace (Qur'an 4:173)

Surely Allah has bought of the believers their persons and their property for this, that they shall have the garden; they fight in Allah's way, so they slay and are slain; a promise which is binding on Him in the Taurat and the Injeel and the Qur'an; and who is more faithful to his covenant than Allah? Rejoice therefore in the pledge which you have made; and that is the mighty achievement (Qur'an 9:111).

On the other hand Allah has threatened those who persist in His disobedience with severe punishment. It is truly up to Him to either punish them or to forgive them:

These are they who buy the life of this world for the hereafter, so their chastisement shall not be lightened nor shall they be helped. (Qur'an 2:86)

Surely those who disbelieve and die while they are disbelievers, these it is on whom is the curse of Allah and the angels and men all;.  abiding in it; their chastisement shall not be lightened nor shall they be given respite (Qur'an 2:161-162)

Surely those who conceal any part of the Book that Allah has revealed and take for it a small price, they eat nothing but fire into their bellies, and Allah will not speak to them on the day of resurrection, nor will He purify them, and they shall have a painful chastisement (Qur'an 2:174)

As for those who disbelieve I shall chastise them with a heavy chastisement in the world and the Hereafter; and they will have no helpers (Qur'an 3:57)

If You should chastise them, then surely they are Your servants; and if You should forgive them, then surely You art the Mighty, the Wise (Qur'an 5:118)

If we ponder on these articles of faith, we see that they are interrelated with each other. If there was  no Day of Judgment, messages of the prophets would have no effect. If there were no prophet, none would know about the Day of Judgment and Allah, and if there were more than one god, everything would be in complete disarray:

Never did Allah take to Himself a son, and never was there with him any (other) god-- in that case would each god have certainly taken away what he created, and some of them would certainly have overpowered others; glory be to Allah above what they describe! (Qur'an 23:91)