I'm 13 years old, 5'1 & I weigh 136 pounds. I don't look as big as I sound. I've played soccer since I was 5.?

I have very muscular legs, and a pretty big butt. I'm very insecure about my stomach though, even though I'm not fat, I would like to get a toned stomach and butt. I know I shouldn't worry about this stuff at 13 years old, but I'm not looking to be anorexic, I just wanna lose a little stomach flab, tone my stomach & my butt. Most importantly, I want to stay in shape. My family hasn't had the best health &I want to start to protect mine early. Anyone have any advice?