IMHO, over twist is better than under twist. If you over twist you can loosen it up by spinning the opposite direction. Plus, the yarn is still knitable. If you under twist then all it's good for is needle felting. The yarn singles fall apart. And even the really good spinners still over twist the yarn. Plying and setting it will help this.
When you learn to draft thinner it may work itself out. Thinner singles need more twist.
Forgive me if I babble on. I didn't have anyone to help me spin. I had to figure it out solo. So the silly little things give me the most probs. Just keep practicing. It will take a while to be really good, but it is sooo worth it. And the yarn lumps and all will look better and better.
BTW, what kind of wool was in your starter pack?