If you want to use your computer to make music, you have a couple choices.
- You can usually find a cheap microphone to plug into your input jack, and you can record basically anything this way. Or you can use a mixer to bring in multiple input sources, and connect that mixer to your input jack or USB. This is a good set up for a full band, so that each instrument and voice gets their own dedicated channel.
- If you have a keyboard with MIDI capability, you can plug it in via your USB jack, and use a piece of software to generate the sounds themselves, like Garageband.
The sky's the limit - have fun!