Find a boxing gym. Learn how to throw a REAL punch. Spar a couple of times. Then you'll know what to do if it happens again. You may find that you don't feel the need to fight this girl to keep your self esteem (after you know she doesn't have a chance in **** of beating you).
buy a gun///.... for self defense
This is definitely not for you!!!
Napalm her house
How could doing that lead to anything else but more trouble for everyone?
get people who will help you and will stand up for you join a sports team thats a good way of making comrads