Our muscles and brain run on carbohydrates like potatoes, pasta, corn, beans, fruits and vegetables. We certainly know we need to have a balance of protein to also make sure we keep our muscles strong and some fat to make sure our bodies and nervous system run correctly. This would be ideal. An example of this would be a bagel with a little peanut butter and apple juice or fruited yogurt and some crunchy pretzels. This would provide you some carbohydrates for your muscles to use during practice and hopefully it would be digested easily so it doesn't sit in your stomach.
Some gymnasts prefer a fruit smoothie before workout.
After workout, it is just as important to make sure you replenish your muscles with fluid like fruit juice, milk or water and carbohydrates and protein like a bean burrito or bowl of cereal with a banana. Fat is always an issue to watch. The best guideline would be typically to keep fat low by reading labels and eat under 35 grams per day. As you are still growing, we want nutrition for building, too. I hope this helps.