To be able to purchase a house, you must be at least lvl 15. When you reach the appropriate lvl you will be prompted to see the guy standing just inside the door to Ravenwood...sorry I don't remember his name. So if you have already reached lvl 15 then maybe it's just that you need to go talk to him to be able to purchase the house. If you haven't reached that lvl yet then you have some work to do to accomplish this. Hope this helps you out some.
