How to loose stomach fat. I’m 17?

I’m seventeen, fairly thin. 101 pounds. But i have some extra flab on my stomach. I don’t want to be anorexic skinny.. i’m happy with my curves. I just want to have a nice stomach… Will running, or going for walks help this or no? I know sit ups will help, but i have a very bad back so it hurts to do sit ups. What can i do to help this, in a healthy way without pills etc.