Help with fighting please?

ok i got jumped one time and beat tha three kids but one had a gun so i let them take my stuff other then that i never been in a fight but i can fight so theres this kid who was trying stuff with my girl and i wana whoop em (his skinny as ever weak) and im 16 hes i believe 16 or 17 now i want to know this would be my first offense how much trouble would i get in if i fought him or least punched him in tha face one time?