As a bike fanatic(my wife says), I have owned, worn out, repaired, built and upgraded all sorts of bicycles. From top of the line $5000 full suspension mountainbikes to the cheap department store bikes. I currently have a road bike, 29" hardtail mountainbike and a seatless single speed trials bike.
IMO, if you are just looking to start out, go buy a department store bike. Then ride, ride, ride. In the future, if you decide that bicycling is your thing, go to a dedicated bicycle shop and buy a quality, upgradeable bike that is sized and priced to fit you. Remember... Even the best bike $ can buy will still make you miserably sore if your ride outside your fitness level.
Specialized, Trek, Diamondback, Giant and many others offer entry level, upgradeable mountainbikes for around $500. Airborne Bicycles is a direct to customer bike company that has some really nice bikes for less than $500
Airborne Bicycles.
Of course though, if $ is no object, skip the dept store and go straight to your local bike shop. They will gladly put you on a machine that would blow your mind.... And lighten your wallet.
If guns cause death, then spoons cause obesity.