Peer to peer networks work very similar to search engines in that they rely on keywords to display search results, but those keywords are solely found in the name of the file itself. So, it should be obvious, the name you give your file will determine how effective it is as a promotional tool. If nobody has ever heard of you before, have your file names contain the names of other bands or artists that are very well known and that are similar to your music. Be sure to include your band name as well as your URL. For instance, if you're in a band named "The Scooby Doos" and you sound a lot like Sound Garden, Tool and Metallica then your mp3 file name might look like this:
Putting your files onto the peer to peer networks is easy. Simply download and install the software for that particular peer to peer network. During the installation you will be asked which folder on your computer you would like to share from; anything that you put in this folder will be available to all the other members of the network. Simply put your files into this folder and leave your computer on and connected to the network for as long as possible (weeks if you can).
The peer to peer networks also use a form of ranking to determine your placement in the list of search results. This rank is determined by how many other computers have your exact file (not renamed or altered in any way) and are currently sharing it. So the next obvious step is to get all of your friends and family to share your files too.