Whether you're demoing a song for your band or recording a masterpiece to share on Soundcloud, you'll likely need a couple of things to connect to your computer. If you're planning on having any real instruments or vocals, you'll need some sort of audio interface to turn your analog sounds into digital ones. I have an M-Audio MobilePre USB for that task, which runs about $180 on Amazon. In addition, you probably want to have a MIDI controller, to "play" all those sounds you don't have real instruments for. These can typically cost $250 - $500 or so, depending on features. At $300, IK Multimedia's iRig Keys I/O 49 comes in at the lower end of this bracket.