I'm sure you look great. Of course there are ways for you to improve your confidence, such as the gym.
Trust me, I used to have absolutely 0 self confidence, i hated everything about myself and would get picked on for how i looked, getting called ugly, and eww, etc.
Fast forward a few years, I've taken more pride in myself, regularly go to the gym, skin care routine and I have changed the way I think about myself. These little steps can go a far way. Now with going to the gym and the time I have gone, I am in better shape than most people my age, and get a few compliments too on my appearance.
The best way to view it, is realise that your time on this planet is finite. You need to make the most of what you have. Feeling down about how you look sucks, however, completely disregard the negative criticism what people give you, especially considering the boy was overweight. He was probably jealous and insecure himself.
My advice to you is change the way you view yourself. It sounds difficult now, but trust me, taking those little steps will go a far way. One of the ways which will massively improve the way you view yourself is a gym membership. Yes it sounds scary at first, with not having an idea on what to do, but that is where youtube will help.
If you get a routine set up for the gym, which you can find plenty of ones online, and that you regularly go, along with eating properly, you will start to notice differences emotionally and physically.
Trust me, I'm sure you're not ugly, ignore the haters. I was one of the hated once, now I feel great and confident with my appearance