You find the song on your computer- under "My Music", assuming you have it saved. Then right click on the icon and choose "Send to". Hold the mouse there and choose "mail recipient". This will then put the song in an email for you, ready to send.
I use yousendit a lot and there's also this one http://www.dropload.com/ sometimes a server is running slow so I opt between either. If its a larger file (like a whole cd) then there are other free websites that would cater for it, but it always pays to save to a zipped folder and send it that way else your puter will take all night to upload and the friend won't want a download taking forever.
First, make sure it is an mp3 or FLAC file and not a wav. or cda. (size issues). Then, upload it to verzend.be and tell your preferred recipient where to find it. Then run and hide from the politicos who think they should be able to tell you what you can or can't do with your own property, despite the fact that you were the one ripped off by the record companies when they charged you a fortune for a cd single which is legitimately yours to do with as you wish... don't start me on that!