What can I do to stop myself from getting fat?

I used to be fat as when I was like 10 and I started to grow out of it because I got tall quickly. I'm 16 years old and I've noticed that I was chubby by my belly but I didn't think of it much. Now I've noticed I look chubby when I look in the mirror. I know it sounds stupid because I would've noticed but I've only noticed now... no one has pointed it out really. Well, i don't want to end up getting fat, what can I do to lose some weight? What should I stop eating and what should I eat more? Exercises?
What can I do to stop myself from getting fat?
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