Buy something cheap and resellable. Your best bet is to buy any bike you can find at a good price and in seemingly good mechanical condition. Cosmetics are not nearly as important. Ride the wheels off of it for a year, learn all the basics of new riding, meet riding friends, take a riding course or two, and wear your gear. The year after, you will have a much better understanding of what type of biking you enjoy and what type of bike suits you best. Then, if you want to sell the bike you got at a good price and buy the bike you didn't even know you wanted.... go for it.