I have gotten a few of my own bands song on to the internet.
First I had to get it recorded... either in the studio or mic up a jam in the practice.
Then get it on to your computer I have 3 recordings 1 done just Mic up in a practice and 2 done in a studio.
once on the computer convert it to mp3 and then upload it... I used a website called Fandalism - Free MP3 Hosting
In true Blue Peter tradition (its a uk childrens T.V. program)
Here is some that I have done earlier.