I slapped this girl and i sooo sory please help me people will think i am stupid and my rep will go down advic?

Talk to a trusted adult about your temper and where you could get some help dealing with it. You cannot go thru life dealing with your issues by hitting someone no matter how much they might deserve it. You did a bad thing, however you see that it was wrong and that is a positive thing. If this gets around at your school, you will need to be prepared to apologize and accept you messed up. It will quiet down, but mainly you need to learn how to deal with strife and anger.