Once you're signed in, there's a "Quick Add" menu on the right side of your screen under your name. Click that and select "Music".
You'll get a pop-up where you can either pick an MP3 off your own computer, or enter the URL of an MP3 hosted online.
If you want to include the artists' information, click More Options and you'll get a page that lets you enter Artist info, album names, artist websites and all that jazz.
If you get stuck, ask me and I'll be happy to help.
Extra note! If you want your music to show up on the main playlist on the homepage, you have to check the box that says "For Everyone" when you upload it!
Thanks!