Becoming a Muslim is fairly easy. You say a few words and you are Muslim. But before you make such an important decision, you must have the following beliefs and understanding:
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 himself/herself out of the folds of Islam; they would have to renew their shahadah. It imperative
That the Holy Qur'an is the true, unadulterated, and uncreated words of Allah. To deny, reject or disagree with any verse in it takes you out the folds of Islam.
True Islam of Ahlus-Sunnah wa’l-Jama’ah (The people of the sunnah and the community) is based on four primary sources: 1. The Book of Allah: the Qur'an, 2. The Sunnah: The ways, sayings, and approval of Prophet Muhammad, may Allah bless him and grant him peace, narrated in authentic ahadith, 3. Al-'Ijma: The agreement of the Ulamah (the scholars) who have reached the highest level of scholarship. Last but not least, 4. Al-Qiyas: Deductive analogy (this is not a source in Aqidah, but it is in Fiqh).
The Shahadah is the testification of faith in order to become a Muslim. You say the following with a sincere heart:
I bear witness that there is no such a thing as a god EXCEPT ALLAH and I bear witness that Prophet Muhammad, may Allah bless him and grant him peace, is the last messenger of Allah.