How to improve control of hand whilst drawing on tablet?
This might seem like a cop-out answer, but... practice! Practice is the main way for you to improve your control and line quality. I've been using a drawing tablet for 10 years and I still make mistakes.

The key to drawing lines is to use long, quick strokes rather than trying to hold your hand steady while you trace your outline. Each stroke will likely take several tries, so don't feel bad if you don't get it right away!

If you find you need further assistance, software like Lazy Nezumi will stabilize your strokes, giving you much smoother lines in general. 

Lastly, as @whrrgarbl pointed out, make sure that your tablet is calibrated properly and that your drivers are up to date.