How to stay fit and healthy?

I'm 15 and most people typically refer to me as skinny. I am a very picky eater and haven't got my period yet. My mom always tells me to eat better or else I'm going to get fat once I get my period since my metabolism will slow down. Plus I'm 4'11" so the weight shows quickly. What are some healthy foods that I can make myself for lunch and breakfast? Also, I don't have access to my gym since I'm not 16, so what are some exercises I can do?
How to stay fit and healthy?
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