Trying figure out a diet and exercise plan for the summer. i will choose best answer?

I am doing marching band for my 3rd year and I remember my 1st two years i was always tired and feeling bad because we practice for a long time and its in the summer so its also hot. I did recently stop drinking soda and now i drink the fruit juices that you would get at food lion. i am still in high school so im living at home and mostly just eat what ever is for supper and during the day ill heat up something from the frig. i know i need a better eating plan. for snacks during the day what should i get? would the quaker oatmeal bars be good? or im not shure but please help ^_^ i really need it.