There's several reason why your butt might hurt. Cycling shorts will help, but they're more about chafing. You may need to change your bike seat. They wear out, and often bikes come with cheesy seats anyway. You don't need a hundred dollar seat for the distances you ride, you can get nice ones for $25 or so. It depends what you want.
Do you have a bike shop where they're nice to you? They can help you out. It may be that you need some minor adjustment to the bike.
Come back and ask all the questions you want. We're here to help.

missliz