Know when to swing,
Know when to throw,
Know where to throw next,
Steal more bases,
Enhance pitch recognition,
Improve fielding skills,
Increase the ball velocity off the bat,
Control the running game,
Locate pitches more consistently,
Improve timing, balance and confidence,
Try goin to a baseball camp. If you are going to do it on your own, I would focus on fundamentals, and getting good workouts in.
practice, but practice the right stuff. go to youtube and look at examples of everything you are interested in mastering.