http://audacity.sourceforge.net/
edit sounds you find on the internet to make them your own or synthesize your own from scratch.
When you do get money, (or if you go to a college/highschool/library with a public computer lab that has this software) I highly recommend GarageBand. It has a library full of a ton of different sounds, a nice visual interface that's easy to learn/use, and is really good for making musical arrangements. The only major downer to it is that its only for Macs.