Reduce the distance, you have a natural mouth guard so don't worry about your teeth, get good using your left arm, the quickest way to end a fight is the liver shot, watch Hajime no Ippo, I used to fight a lot as a kid and learnt a lot watching it, also get good on the ground.
Don't be intimidated by bigger people, they are usually the weakest, they never learnt jack shit and use their size to intimidate, opposite can be said, don't underestimate the smaller fighter.
If you're over the age of 18 don't fight unless it's for self-defence, it's not worth it, legally take out your phone and record them assaulting you, then go and give them the beating of their life.