For the player who is serious about this sport, I mean very serious, an athlete's diet follows the seasons. There's a diet for each season in baseball - offseason, preseason, prime playing season, and post season competion. The key to those seasons is the competitive side of this sport. Now granted, if you're a multisport athlete, then it would follow that each specific sport would have it's own demands, thus intake tailored for that sport is your guide.
CAUTION: for someone of your age who requires a balanced diet necessary for proper growth, body functions related to the heart, your nervous system, healing, disease inhibitors and so forth, narrowing your diet to strictly non-meat can be a challenge and not to be taken lightly. Don't self teach yourself here. Professional advice and guidance is mandatory from a Registered Dietitian Nutritionist. However, not every Nutritionist is a Registered Dietitian. These Registered Dietitians are expensive, but you get what you pay for. When working with these professionals, I was surprised at how many mature men were absolutely ignorant of proper eating habits - hence, by the time these men reached their early 30's, many were physically challenged to keep pace with much younger men.
Your health history, your availability to reliable and consistent food sources, the finances available to you to sustain those ongoing food sources, your dedication to stick to a vegetarian diet - even when away from home, packing your own school lunch and possibly being at the mercy of jokers and others who may want to make fun of you decision, and a host of other unforeseen situations will definitely test your will. .