One becomes a Muslim by believing in the Two Testifications of Faith and uttering them with the intention of leaving out blasphemy.

What are the Two Testifications of Faith?
The Two Testifications of Faith are:

In English

I BEAR WITNESS THAT NO ONE DESERVES TO BE WORSHIPED EXCEPT ALLAH and

I BEAR WITNESS THAT MUHAMMAD IS THE MESSENGER OF ALLAH [AUDIO]

In Arabic

ASHHADU ALLA ILAHA ILLALLAH

WA ASHHADU ANNA MUHAMMADAR-RASULULLAH [AUDIO]

The non-Muslim embraces Islam by believing in the Two Testifications of Faith and uttering them with the intention of leaving out blasphemy.

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 ^Abdullah 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 (Allah) only with the acknowledgment of the message of Prophet Muhammad.