Healthy Eating. :)

Hello. A girl of 15 writes to you. I am 172 cm tall and weigh 61 kg. About two / three months ago I was 55 kg. But from eating a lot at Christmas and New Year, I gained weight. I am still in the norm of healthy weight. I don't want to lose so much, maybe only 3/4 kilograms, but I want to tighten my body. Mostly to make a flat stomach and shape my butt. And here comes my problem. As a child I was always pampered and for something good if I did they bought me chocolate, chips, something else unhealthy. From there I got used to eating like that. And now it is very difficult for me to limit myself from "bad foods". I know about training. :) I have already started training. But do you still have any ideas what to eat for breakfast, for example, to stay full for at least a few hours ... Another problem for me is school. It is very difficult to eat healthy while at school. If, for example, I am on the first shift, I have to leave home at 7. And although I have breakfast, I can't eat anything until 2 o'clock - until I get home, because I'm hungry, but there are only pizzas and toasters. And in your opinion, if I train 3/4 times a week and eat well, will I be able to tighten my body by summer? Thanks to those who read it! :)