Becoming Muslim is fairly easy. You say a few words and your Muslim. But before you make such an important decision, you must have the following beliefs:
That there is no god worthy of worship except Allah who doesn't have any partners nor helpers. The one who utters this statement, he/she understands that Allah is unlike any of his creation.
Allah sent His last and final Prophet, Muhammad, may Allah bless him and grant him peace.
To believe that Allah sent prophets, messengers and anyone claiming to have communicated with Allah after Prophet Muhammad, may Allah bless him and grant him peace, takes him/her out the folds of Islam.
That the Holy Qur'an is the true, unadulterated, uncreated words of Allah. To deny, reject or disagree with any verse in it takes you out the folds of Islam.