Imam Ash-Shafi`i said, "That a servant meets Allaah with every sin except Shirk is better say than meeting Him upon any of the innovated beliefs."
WHAT IS SHIRK? Skirk شرك is essentially belief in more than one God, or associating partners with Allah (subhanahu aa ta'ala) by attributing the powers of Allah (subhanahu aa ta'ala) to anything of His creation be it person, angel, place, or thing.
CATEGORIES OF SHIRK شرك
Shirk in Rubūbīyah (Lordship)--- This category of Shirk refers to either the belief that others share God's Lordship over creation as His equal or near equal, or to the belief that there exists no Lord over creation at all.
Shirk in al-Asma was-Sifat (the names and attributes of Allah)---Shirk in this category includes both the common pagan practice of giving God the attributes of His creation as well as the act of giving created beings God's names and attributes.
Shirk In al-'Ibadah (Worship)---In this category of Shirk, acts of worship are directed to other than Allah and the reward for worship or asked for aid is sought from the creation instead of the Creator.
DEGREES OF SHIRK
Ash-Shirk al-Akbar (Major Shirk) remove you from the fold of Islam. This form of Shirk occurs when any act of worship is directed to other than Allah. It represents the most obvious form of idolatry which the prophets were specifically sent by Allah to call the masses of mankind away from. Examples of this shirk include asking forgivness paradise and other things that only Allah can from other than Allah.
Beware also Ash-Shirk al-Asghar (Minor Shirk) [these you can be forgiven for but they lead to major shirkشرك]: Mahmud ibn Lubayd reported, "God's messenger said: "The thing I fear for you the most is ash-Shirk al-Asghar (minor shirk)." The companions asked "Oh! messenger of God, what is minor Shirk?" He replied "Ar-Riya (showing off), for verily God will say on the Day of Resurrection when people are receiving their rewards, 'Go to those for whom you were showing off in the material world and see if you can find any reward from them." Mahmud ibn Lubayd also said, "The Prophet came out and announced, 'O people, beware of secret Shirk!'
The people asked, 'O messenger of God, what is secret Shirk?' He replied, 'When a man gets up to pray and strives to beautify his prayer because people are looking at him; that is secret Shirk."
Before you study anything else within the subject of your deen, Islam, you want to study first Tauhid (the idea of the Oneness of Allah (subhanahu aa ta'ala)) and know your Aqueedah (your faith in the form of practice of Tauhid) to protect yourself from the worst of all sins----shirk.
As soon as you tell people you are a Muslim they are going to start asking you what kind of Muslim you are: sunni, shi'a, sufi, Ahmadiyya, Ismi'li, Wahabi... and you're gonna go, "didn't the Qu'ran tell us not to split ourselves into factions?" because all you know at this point (hopefully) is the Qu'ran. Then people are going to try and tell you their version of Islam and give their daleel (Islamic evidence) for why you have to do things certain ways. If you are not strong enough in your Aqueedah and the idea and precepts of Tauhid, there is NO point whatsoever of you getting into Shariah (Islamic law), Fiqh, or trying to make sense of different scholars opinions. The key to what is truly right and wrong in Islam is Tauhid---belief and dependence in/on One God, and understanding everything that can negate that belief. If you are strong in your Aqeedah, inshaAllah none but Allah can lead you astray.
One narration states, "You [Mohammed] are going to a people from the People of the Book. Let the first thing that you call them to be the worship of Allah. If they acknowledge Allah, then inform them that Allah has obligated upon them five prayers during their days and nights." [Al-Bukhari, Muslim]
This hadith is clear. It does not require much of an explanation. The Prophet (peace be upon him) applied this principle in his practical calling to Islam. He stayed in Makkah for thirteen years to teach the people iman in Tauhid and to educate his Companions on this point and to correct the beliefs of the people. That is the pattern upon which the Companions were brought up.
Jundub Ibn Abdullah al-Bajaly said, "We learned iman (faith) and then we learned the Quran and it increased our iman."
Abdullah ibn Umar said, "We lived during an instant of time in which one of us would receive faith first before receiving the Quran and when the surahs were revealed we would learn what they permitted and what they prohibited and what they forbade and what they ordered and what should be the stance towards them. But I have seen many men from whom one is given the Quran before iman and he reads it from the opening of the Book to its closing and he does not know what it orders and what it forbids and what should be his stance towards it. He is like someone who is just throwing out dates [i.e., he does not get any benefit from his recital]."
That is the manner in which the Prophet (salla Allahu alaihi wa-sallam) brought up his companions: Iman first and then the Quran. This is similar to what Imam Abu Hanifa pointed out: Understanding in the religion first (i.e. tauheed) and then understanding in the science (i.e. the shariah).
The beliefs must be corrected first (Tauhid in Aqueedah), then follows all of the other aspects of the religion....
Source: Girls Guid To Being Muslim
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيمِ