Do you enjoy sitting around and playing songs on your guitar or piano? You have fun, but you're not quite good enough to play professionally?
That shouldn't stop you from recording it and selling it. There are a lot of things you can do to make some money off your music.
For one, you can simply play a song (ideally, one you've written yourself to avoid copyright issues) and list that song on online music stores. You can also make a video for that song and post it on YouTube (with ads enabled, of course). That way, you can make at least a bit of money from your recording.
For another, you can give lessons, teaching others how to play. You can advertise on places like Craigslist for music lessons face-to-face, but you can also do things like record videos of your lessons and post them online.
Finally, if you live in a city or a town with a compact downtown, you can try busking performing in the street or a subway station. (Subways or narrow streets work best if you don't have an amplifier.) Tourists and commuters alike may appreciate the ambience, and tips of a dollar or quarter at a time can add up quickly. At the worst, you'll get some valuable practice performing in public; but you can make some decent money, too. Just remember to research whether you need a permit to play in a certain spot.