What to do?

I can't stand my fiancé baby momma. She always is constantly trying to start stuff with me. I've already beat her ass once and I don't know what else to do. I'm 17 she hit me first then I went in on her she is 20yrs old. This was all over her stealing 14$ from us, and claiming it was "child support". We haven't went to child support court yet and I'm not sure what else to do. I need advice.