Through the acceptance of the Shahadah(a testimony of Islamic belief), Muslims acknowledge that Allah (سبحانه وتعالى) is the Creator of all things, that His(سبحانه وتعالى) hand is the dominion over all things.
Hence, we Muslims bind ourselves by worshiping Him(سبحانه وتعالى) alone as He(سبحانه وتعالى) is indeed the only diety to be worshiped.

What responsibilities does accepting the Shahadah entail?
What actions must we undertake?