I tried to throw a change up but it kept on hitting the ground. My coach said dont throw curveseither.
In response to you blackdragon......You didn't state your aim location....you might already know that the change up will drop, so you want to aim at your catchers nose.......unless he line up straight down the middle, then you'd want to throw it at his left or right ear, to avoid the ball being hit straight at you. So by aiming you pitch higher, you will get the sink you want right over the plate.
Also, I agree with your coach to avoid throwing curveballs....save that for more advanced pitching, when your arm is fully developed. You might want to stick with just a 4 and 2 seam fastball and your change up.
Lastly, to address your soreness.....you don't want to be sore in your shoulder or elbow joints, that's means your mechnics are bad. Perhaps try keeping you arm long (by staying back on your back push-off leg a tad longer to be able to extend your arm out before your forward movement) That way you're not getting your body in front of your throwing arm, which will cause soreness where you don't want to be sore....in your joints. If you're sore more underneath your arm, on your body.....or the back of your shoulder- then that's a good sore. You're building up your muscles then. But make sure you're not sore anymore before you go back to pitch....it needs to rebuild the muscle tissue.
I hope that helps and good luck.