It's different from office to office. In some rare cases the dentist says that he is willing to train the right candidate, then you're really lucky. That's what happened in my case, I guess I impressed him somehow because he was willing to teach me everything.
You really just have to be patient. Most dentists want assistants that have experience, and you can't really blame them. But there are some that are willing to put in some work to get an assistant that works to their liking. You could also start by shadowing and showing the dentist how involved you are. Pay attention to everything the assistant does and if there's ever an opening, as for the position. Good luck!