When it comes to playing baseball, talent is what you have, but effort is what you give. You may not be born with the ability to hit homers like Albert Pujols or have the blazing speed to steal bases like Rickey Henderson, but with dedication you can increase the skills you do possess to make yourself a valuable asset to any team.
Dedicate yourself to the game of baseball. In order to excel at anything, you need to have a passion for it. Spend time watching baseball, reading about baseball, collecting baseball cards, watching movies about baseball, naming your dog after your favorite baseball player.... True, none of these things are going to increase your baseball skills on their own -- there are plenty of superfans out there who never actually play the game. What this passion will do, however, is give you the drive you'll need to keep going with all of the other, less-fun parts of working at your game.
Practice every single day. Drills aren't just for when you're with your team; you can practice solo drills any time on your own. Throw a ball up in the air, then catch it. Throw a ball against wall or a stoop. Play catch with a friend or sibling. The best ballplayers are always doing something that will help them improve their skills.
Use a batting tee to help you straighten out your swing. Some players object to using a tee as they think it's just for little kids, but this is not true. Even major leaguers will still practice off a tee in order to perfect their swings.
Talk with a coach or a trainer to develop a strength-training routine. Strength training is recommended for every athlete from about age 14 on, and a good coach should be able to help you find a regimen that will be right for your size and the position you play. After a few weeks or months of lifting, you should be able to see a noticeable improvement in the power and velocity of your hits, as well as in your endurance.
Play up to your level, not down to it. Resist the temptation to stay on a team where you are the oldest, biggest, most experienced player, and instead take the opportunity to “play up” and join a team where you may find yourself one of the younger, smaller, less experienced team members. Sure, you will no longer be the “star,” but you will be learning and improving your own skills at a much faster rate than you wold if you stayed on a lower-level team.