How do I defend myself in a school fight?

I am an 8th grade middle schooler. Today when I was at P.E. I was went to get a drink of water and this random 7th Grader comes up to me and slaps me. When he turned around and started laughing I slapped him back because I was bigger than him and I was pretty sure that I could take him, but then he went crazy! (He must have had a major case of ADHD or something of the sort) He pinned me to the wall and starts punching me in the stomach. My heart started beating extremely quickly and I got very nervous so I couldn't do anything back to him. Then his friends came and started beating me up as well, I was helpless, nobody was around. Then they went away laughing while I was bruised all over. At lunch I got them back by going up to them with alot of my BIG friends and threatening them not to ever touch me. That got settled but I would still like to know how to defend myself. (It's my nervousness which takes advantage of me when I am in a fight.)