That's a wide open question there, Tom! What is it that you want to improve? Figure that out and you're half way there.
In the mean time, do crunches and push ups every day before school and before bed (or whenever...but do them regularly and every day), and eat well, get plenty of rest. This will enable you to be athletic enough to work on specific skills. The idea is to work the core (abdomen and back) muscles, because that is where your stabilizer muscles are. Everything functions athletically from there.
There are basic exercises that are good for agility and conditioning, and weight training to improve strength. Talk to your coaches and/or the high school coaches to see if they have a program available or if they can pin point some specific areas for you to work on.
Take a few minutes to decide what you want to get better at. If it is "everything", then break down the specific needs and create goals from them. For instance, since you posted on a pitching forum, I am assuming you want to get better at pitching. Do you want more velocity on the fastball? (who doesn't). Do you need to improve your control? Off speed pitches? Or is it the mental/ tactical game that you want to polish up? Figuring that out will get you started. Then, post specific questions or concerns to the forums and it will be much easier for us to help and for you to get solid and helpful information.
Go get 'em, man! Just the fact that you want to improve shows that you're competitive. Thats a good place to start.