How tall are you?

Bikes, like shoes, come in sizes. You might be able to physically ride a bike that's the wrong size, but it will never feel right and, if it's really your first bike, you might not even know what "right" feels like. One of the nice things about buying a bike from a shop is you can pick out the style and price range that you're comfortable with and depend on the shop to get you the right size.

You can save a ton of money buying used but there's usually only one bike to look at. Unless you already know it's a size that fits you it's easy to make a mistake.