One becomes a Muslim by believing in the Two Testifications of Faith and uttering them with the intention of leaving out blasphemy (kufr).
What are the Two Testifications of Faith?
The Two Testifications of Faith are:
أشهد أن لا إله إلا الله وأشهد أن محمدا رسول الله
In English: I BEAR WITNESS THAT NO ONE DESERVES TO BE WORSHIPED EXCEPT ALLAAH and I BEAR WITNESS THAT MUHAMMAD IS THE MESSENGER OF ALLAH
In Arabic: ASH-HADU ALLAAILAAHA ILLALLAAH WA ASH-HADU ANNA MUHAMMADAR-RASOOLULLAAH
The non-Muslim embraces Islaam by believing in the Two Testifications of Faith and uttering them with the intention of leaving out blasphemy. One may utter the testification of faith in either Arabic or English. If one cannot pronounce the letter (Haa’) correctly when saying “MuHammad,” then one says “ABAL-GAASIM” instead.
MEANING: The first Testification means that no one or thing deserves to be worshipped except God Who is the Creator of everything. The second Testification states that Muhammad the son of `Abdullaah who was born in Makkah approximately 1400 years ago is a messenger of God. He was sent to convey what God revealed to him so that the humans believe in and work with the Laws he received and brought. He is truthful in all what he conveyed. The Two Testifications reject the godhood of anyone or anything other than God. They confirm the Godhood of God (Allaah) only with the acknowledgment of the message of Prophet Muhammad.
WARNING: Beware of some ignorant people who delay a person from becoming Muslim. Telling another person who is requesting to become Muslim to for example “wait” or “come back later I am busy” or “go take a shower first and come back” or “go read this book and come back” are all means of delaying ones Islaam. Delaying a person from embracing Islaam when able to tell him or her to utter the testification of faith is in itself blasphemy (kufr)!