I am thirteen and am really good at baseball. I want to know how to get better. Are there any mental states of mind that you need to be in? Could you supply drills to be better? How good of physical shape should I be in? I run the mile in six minutes. Are there any other tips for being better at baseball?
For your birthday or Christmas ask for these books as presents, or if you can buy them now:
* The Science of Hitting by Ted Williams
Best book on how to mentally prepare you to be a better hitter in baseball through the techniques he teaches. When he took over as manager of the Washington Senators in the 60's, with roughly the same core of the team, he got the team to go from .224/.287/.336/.623 to .251/.330/.378/.708 in one year.
* The Mental Game of Baseball by H. A. Dorfman
Sandy Alderson, Tim Belcher, Al Leiter, Jamie Moyer who swear by him and his books. There is also “The Mental ABC's of Pitching” and “The Mental Keys to Hitting” by Dorfman too, whichever you need (you didn't mention what position you played).
* The Baseball Forecaster by Ron Shandler
I think it would help become better if you understand the sabermetrics of how to tell how to identify good players. Read the section on Forecaster's Toolbox, it will explain what a good hitter and pitcher is, using stats. If you understand the basics, then you will understand what you need to do as a player to be good.
For pitchers, you need to be good at throwing strikes (and, key, unafraid of throwing strikes and letting your fielders contribute) and avoiding balls, simple as that. For hitters, it is the similar only from the hitter's view, you need to be good at swinging only at strikes and avoiding the balls, plus when you get the strikes, as Ted Williams advocate, you need to take advantage when the pitch is in your zone and drive it for extra-bases, particularly the homer. There are other things which you can just apply to yourself to make your play better. Just use the rules there to give you goals as a player and work backwards to being a player to see what you need to do to achieve that good stat.
Lastly, I think part of Ted William's success was related to his being a Marine and being trained on how to make yourself physically at your peak. If you can join some sort of fitness bootcamp that is modeled after the Marines, I believe that would be good to go through and keep up. The best players today keep themselves in tip-top shape year round now, plus this should add muscle so that you can hit pitches harder or throw pitches harder.
However, don't measure yourself by conventional fitness terms. Being able to run the mile in 6 minutes is good, but that doesn't really translate into baseball exactly. Find out from a trusted coach what type of exercises you should be improving at to get into baseball shape for the position you play.
But I think the bootcamp is good for building overall stamina, as baseball is a game of attrition, in the majors, there is 162 games, and people tire out. If you can improve your fitness to that of a Marine, you will be at your peak longer than others who tire out sooner, so even if they are better than you, they won't be if they tire before you do.
All the books are available relatively cheaply on Amazon.com. If you want to save 50% or more on Shandler's book, you could get an older version of it from BaseballHQ.com, the toolbox is basically the same each year, with new additional research added each year, incrementally improving. I think the older book would be good enough for your purposes.
Good luck, all the best!