How much you control in the dream will vary from dream to dream. Dreams are your brain's fucked up way of running a little reality simulator at night, and lucid dreaming is the art of realizing you're in a simulation and hijacking the program.

In any dream, lucid or not, basically what happens is whatever your brain expects to happen. Sure, there are some random things that you don't expect but for the most part if you expect something is about to happen it will. Lucid dreaming, therefore, means you have to overcome your expectations and learn to consciously control them.

This is difficult, because lucid dreams are so stunningly like reality in every detail that sometimes you doubt whether you're really dreaming. If you're on the roof of a large building, and everything looks and feels exactly as if you were on top of a large building, and you decide you want to fly, you have to overcome the very strong expectation your brain has that you will plummet to the ground. You have to first be certain that you are dreaming, and then be certain that your arms really can be used to counteract the force of gravity. We're not used to making ourselves believe things we know to be false, but this is what you have to do if you want control over your dreams. If you do it well, you can be a god in your own little customized universe; if you don't, you will be just a curious spectator who knows he is dreaming but can't do anything about it. Just remember, in lucid dreams, there is no spoon.