Imdeanna is right to advise you to test ride bikes to see which ones fit you and are comfortable. Many Local Bike Shops (LBS) have used bikes, too. While alot of people test ride bikes at bike shops and then buy the bike online, it's not really a good habit to get into. The LBS depends on sales to stay in business. If you find a good LBS, treat them well, and they will treat you well in return (with extra services and discounts and rush repairs, etc.)

Shop around and see what the LBS in your area have for sale. Don't disregard new bikes, too, because last year's models are often on sale.