Some very good advice from above posters. As you can see, Defiler is one of those classes where it's very easy to play very poorly. This is true to some extent of a lot of classes, but Defiler seems to really be a good example of it. It's not an easy class to play, but when played well is extremely effective which is why it's my favorite (I have a lot of high level toons).
But things get a lot better later on. Have fun.