How to get muscles during the summer?

I am 5'7" 17 year old 135 pounds and I have never won a fight with a kid over 14 or over 103 pounds and never fought a kid over 105 pounds or over 15. I have lost all 60 fights to kids in high school and I am 12 and 75 aginst kids from the middle school including losing to a 13 year old 5'3" 96 pound kid who never won a fight before in his life till he beat me up and KOd me last friday night.