Craigslist is your friend. Read some sizing guides online to get a baseline for fit, then go try a few out. Wal Mart will get you a $150 bike which will likely be utter crap and weigh a ton. Bike shops will probably start in the ~$600 range for a basic bike, up to ~$10K. You can buy a perfectly serviceable front suspension bike from a major manufacturer on Craigslist for around $2-$300. This will allow you to get some experience and learn what you like and don't like.
This, I'm willing to ride a 21 speed, or 24, bought my son a 24 speed bike that was a top of the line bike 8 years ago for $300 last year and its in great shape. He has dinged it up more than the previous owned had in 8 years.
