Can this help me gain muscle and get a flat stomach?

Hiya! I’m a 13 year old boy and I have lost 90 lbs. The docs want me to stop losing and start to maintain. So I started eating junk and haven’t gain anything cause i’m more active now. But I want to build muscle and have decided to stop eating junk food. For breakfast I’m going to have fruit salad contain grapes, strawberries, and blueberries. Healthy? I exercise everyday I lift weights!