What am I suppose to tell the lady at the dentist what I'm there for?

I just went to the dentist and she asked what I'm here for, I didn't really know what to tell her so I said a cleaning. I thought when you go the dentist just checks your teeth and tells you what he needs to do like clean or fill a tooth or something. Then she said the dentist is only there Tuesdays and Thursdays so I have to go back. My teeth are pretty bad, a little crooked and yellow so I defiantly need a cleaning or something like that..