if you believe in the Oneness of God, that He is alone without any partner, far and removed from having a son or being part of a trinity, totally incomparable to His creation and He alone is deserving of all worship; and you believe that Muhammad is His final Messenger, and you believe in all of those great Prophets and Messengers before him, then you simply repeat the following: Ash-hadu an laa ilaa-ha il-la llaah, wa ash-hadu anna Muhammadan rasoo-lu-llaah (I bear witness that there is nothing deserving of worship except God, and I bear witness that Muhammad is the Messenger of God). That's it!
