Look at a hybrid or fitness bike if by "trail", you mean a paved bike/walking path. Get a bike with 700c tires (as opposed to 26" tires on a mountain bike) for a more efficient ride. Trek, Specialized, Giant, Raleigh and Schwinn all make decent bikes. Visit a few bike shops and start looking/test riding, but dont buy until you look at several bikes. Don't buy a bike at a discount store as they are total junk. You'll probably need a 19" frame, but a test ride is the only way to know for sure.
Used bikes are a risk if you dont know a lot about bikes. Try to find a friend who can help you out. If thats not possible, test ride the bike and make sure the wheels dont wobble. If the bike doesnt shift or brake smoothely, expect to pay about $30 for a bike guy to tune them up, and negotiate that off the price of the bike. Dont worry if the tires are shot; they can be replaced for under $20 each.