How do you learn how to fight?

ok im going to one of my best friends b day party next week. but there's a problem. a girl that's coming completely hates me. its not like i was mean to her i just don't really like her bff and she took it the wrong way. im not missing the party its going to be awesome. anyway if a fight does break out which i hope it doesn't i wanna be able to fight well. i'd probably be able to take her now because shes a prissy snob and a weakling and i've taken her down before. so how can i learn a little really fast?