How to train my shot aiming?

[Daniel]

I have been playing tennis for about four years now, am sixteen and have a one hand backhand, topspin grip forehand. Recently I have been able to hit the ball consistently but when I played in a match, was creamed. I have lots of flaws, I don't play enough and have a lower than average serve and return serve, in addition to volleys being just OK. Mainly a few tips are how to improve my shot selection? Sometimes I want to hit sown the line or cross court and it just beams down the middle and when I want to try a shot like that in a match, the ball spins out of the lines... :/