Dont sweat it, I was in the same boat as you a few months ago. They're just going to teach you proper form and such in your first class. Keep up the boxing though, I'm 15 too and I was like 5'10 and 245 when I started, and I'm already down to 200 just from boxing. If you eat better and start boxing 3 or more days a week you'll shed weight.
Youll learn a lot of new things such as stance how to punch and probably little exercises like push ups or something have fun (:
Stances, make sure to bend the legs and stand on the balls of your feet. Maybe a couple of combos, like, left jab, right punch, left hook, right punch, left uppercut, and right punch with a trainer.
dont kill someone
Im 15 year old are i wanted to do boxing , im 5'7 and 210 i know im fat that's why im going in to boxing and i wanted to know what ima do in the first day
