Simply accept with your mind and heart whole heartedly, believing that and uttering with your tongue that there is no god but god who is worthy of worship and that Muhammad peace be upon him is the last and final prophet and messenger of god.
Understanding that the quran the last testament brought forth for mankind to follow in the hopes of achieving salvation. And that as a muslim ( one who submits to god) you/ we should work hard to serve as well as worship god by following his commandments.
Ash hadu an La ilaha ila llah wa ash hadu wa anna muhammad rasulullah
TAWH33D ORIGIN OF RELIGIONS ISLAM 