Get a punching bag and practice some to get your coordination up and to tone the muscles involved. Also jump rope to strengthen your legs, which is where most of the force of a punch starts. and your calves which help you stay light on your feet to avoid being hit or at least hit as hard.

Besides that, just do a lot of exercise or sports in general to build strength. Most fights among novices aren't won because one of them is especially skilled, rather one of them landed a small # of hard blows because in a real fight unlike boxing, blows to the head or stomach without a glove on can do more damage faster, especially if the other person isn't ready for it.

There is a reason to be scared, that it's an activity for stupid people and they may be too stupid to not injure you, perhaps even permanently if they knock a tooth out.

They probably don't have insurance that would cover an injury to you either so they could end up sent away to juvenile prison which is worse than an adult jail because you have no right to a trial or release until they or your parents decide it.

You will be better off if you avoid people who fight and especially those who try to bring other people into their games. It's not something that adult society tolerates and part of growing up is becoming a member of that society. Otherwise the place to play box is in a boxing ring with gloves on and that same gym that has one, has bags and people who will help you build up skills.

Everyone gets anxious during a fight and everyone is scared of loosing, even the best boxers in the world.

You have a weak left front hand. Try switching to southpaw.

this video will give you the best answer, better than anyone here.