I need to improve my upper body strength. so what should a 13 year old make as their diet?

I'm a 13 year old in 8th grade. and i totally want to improve my upper body everything. I'm a decent runner and I have a lot of endurance on all parts of the body. My problem is, I'm not that strong, I'm just able to take in a lot. I feel like a total lackey out of my group of friends, because they all are so strong and can play basketball or any sport but I can't. I can't beat one of my friends in an arm wrestle, so I feel really dumb. I want to change it. I know there are going to be some dietary sacrifices so let me tell you some stuff about myself right now. I lack a lot of calcium because I don't like milk and I don't enjoy all meats. I need calcium. I eat eggs, lucky charms with milk, or corn flakes with sugar and milk, or a pop tart for almost all breakfast. I also like to have french vanilla cafe every now and then. not often. For lunch, a lot of the times I don't have it. I end up eating a bowl of popcorn a lot, or a bowl of cereal. Dinner I usually have pasta with shredded 4 cheese mexican cheese and broccoli and a meat. I usually come back for a snack. There, i have either a bowl of popcorn, a plate of tortilla chips with shredded cheese on top, lays chips, oreos, or cereal. Those are some of my fav tastes. I also want to know, what should I eat after working out? I like to eat an orange after working out. I also enjoy plums and watermelon. oh yeah, I also bike a lot.