In baseball, you need to build strong fundamentals at a young age in order to become a successful ballplayer when you're older. Here's a few of them I picked up from my playing days that really helped me improve my game.
Hitting: 100% Focus on the Ball
This is something an old coach of mine told me, and it really helped me a lot. The idea is simple, if you see the ball 100%, you will either hit it well (if it's a hittable strike) or take the pitch (if it's an unhittable ball). It may seem silly, but going up to bat with this mindset will help you keep a clear mind, and stay in the moment.
That tunnel vision of you putting 100% of your thought energy to focusing on the ball, really helps you see and track the ball properly, no matter how fast. It also helps you barrel up the ball more, and it makes it easier to pick up on the spin of curveballs, cutters, and sliders. This mentality made me a much better hitter, and I'm sure it will help you too.
Fielding: Know Your Runner
Whether you're in the outfield, infield, or behind the plate, estimating how fast or slow the baserunner you're dealing with will help you prepare for the play. This works in all cases: when the the runner is hitting and running out of the box, a runner is trying to steal from first, or they're standing on third ready to tag up on a fly ball.
For example, if you're dealing with a fast runner in the infield, you'll know you need to charge the ball more and get the throw off quicker on a grounder. You may even have to prepare to make a barehanded play with the ball!
If you're a catcher trying to throw out a runner trying to steal second, it'll let you know how fast your pop time will need to be, and how fast you need to get the throw off. Overall, it lets you know exactly how much urgency you'll need to get the out and make the play. Small details like this separate good fielders from great ones.
Pitching/Throwing: Elbow, Wrist, Ball
No matter how hard you're throwing, or where you're throwing it from on the field, the correct throwing mechanics always stay the same. Elbow, wrist, then ball is the proper sequence your arm must go through to make a successful throw.
It may seem hard to drill and work on this, but all you have to do is think about this and have this framework on your mind when you are throwing. When you're playing catch with your friends, think elbow, wrist, ball. When you're warming up on the line, think elbow, wrist, ball.
Obviously you have to set your feet when you throw and include the other important throwing mechanics, but this mindset helped me make much more accurate throws, and it will help you too.
And like I said before, it may seem silly, but little fundamentals like this really make a difference in your game. I encourage you to incorporate some of these ideas into your practice, and see the results for yourself!