I'm self concious. My stomach isn't flat, and my legs are flabby? Help?

Okay so, I know I'm still growing, but lately, my parents have been buying mcdonalds a lot (mainly for lunch) So, Could my leg flabbiness be a result of that. The only time its flabby is when i sit down. When I stand, It is mainly muscle, except when i walk, my lower leg jiggles a bit. My stomach is also big, and I need it flat! Are there any ways to make your stomach flat. I am determined to stop drinking soda, as well as trying to eat healthier. How do I cope with this? It is really scaring me.
I'm self concious. My stomach isn't flat, and my legs are flabby? Help?
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