Find a field and a fence (or wall.) Put a trashcan at the fence and walk a straight line from it until you get the distance from home to second base. Set up home plate there. Take a bucket of balls. Get in your crouch with one of the balls in your glove (as if you just caught it). At a "trigger" (count down 3-2-1) transfer the ball, move your feet toward the target (can) and throw for accuracy. Work on your movement from crouch to throw. Work on a smooth, quick transfer from glove to hand while moving forward. Think about your footwork and follow-through. Go through all of the balls. Pick them up and start again. Work on making the throw from your knees, just in case you can make it following a block. The big thing will be making your transfer from glove to hand very quick and smooth (work on that over and over without throwing), then your footwork and balance, and using your back leg for power while following through with your body. Don't let yourself stand too high before the throw--stay low and zing it. Practice quickly finding the seams on the ball in your glove without looking. Throwing a 4-seamer will improve accuracy to 2nd.
It may be expensive to hire an instructor. If you can't do that, see if you can find an instructional video or even a book. Watch foot movement, ball transfer, and body mechanics over and over. Practice!
That's it! Great good luck.
Also, try doing some plyometric exercises that help develop your legs. For example: Put a bat on the ground. Practice jumping over it from side to side until you are tired. Add time as you get better. Do the same thing using only one leg-- change legs. Replace the bat with a crate or box when it gets easy. Do lunges/ swamp walking around a track.
