Can someone help me get started. I start by assuming 1-1 and know that i go the other direction, but can't seem to see how to use 1-1 to show anything...Thanks in advance.

Theorem:

Let $\displaystyle \Phi :R \rightarrow S $ be a ring homomorphism. Show that $\displaystyle \Phi $ is 1-1 iff $\displaystyle ker(\Phi) $ is trivial