What can I do to stay fit?

I'm a quite skinny boy and I'm 15 years old. I eat healthy which has always helped me stay fit. However I have noticed in this year that has gone that my stomach is a littleeeee rounder than normal. What can I do to stay fit? I'm not looking for abs but just looking for some exercise I can do to not gain weight by fat (I obviously grow and gain weight but you know what I'm saying) Please also tell me how often and for how long I should do it! Example: "Do sit-ups for 10 minutes 4 times a week"