newbie question

I have a question I was hoping someone could help me with. I'd like to start getting into mountain biking, but I'm not sure what kind of bike to get, or where is a good place to purchase it. I realize there are a lot of different options depending upon what I want to do and how much I'm willing to spend. Currently I have a road bike, so I don't want hybrid or anything like that, I want a bike that I can take up and down trails and have a lot of fun with. Also since this is a new sport for me so I don't want to spend ton of money just to get started.
So what type of bike should I look for, and where is a good place to purchase it?
thanks,
John.