Dieting over the summer?

I'm been trying to lose 20 pounds for the pass few years. My doctor has been telling me I've been at an unhealthy weight range for a while. I'm 15, 125 pounds and 5'1. My self estem has dealt with many blows, leaving me, quiet and alone. Please, is there anything I can do over the summer, to lose 20 pounds and gain my confidence ? Like excerise routines, food choices. I know no junk food and fried food . I don't eat either of those things often. I don't understand why it isn't helping. Please. Help me.