You are a Muslim by simply believing that " There is only one God, Allah, who alone deserves to be worshiped, and prophet Muhammad is his messenger"
Saying this is called Shahada (Testimony), in Arabic, it is spelt as follows:
Ash Hadu Alla Ilaha Illa Allah, Wa Ashhadu Anna Muhammadan Rasul ullah
It also means that you believe in all of God's Prophets, including Jesus, peace be upon him, who is a Messenger of God and not divine nor the son of God.
You should know that by becoming a Muslim ALLAH has promised to wipe out all your previous sins completely, and you will start with a clean sheet.