I don't as much as I used to but in my 20s-30s I was doing it pretty regularly. Started off with a used hard-tail Specialized Rockhopper I found off of Craigslist for like $100 in great condition. Do yourself a favor though like mentioned and visit the bike shops, because you can sit on a few and learn what size you need, etc. Once I knew I was in to it seriously I went and got a nice bike from a shop but I'd try it out first and see what you like and if you like it because some of those bikes get really expensive.
