If you want to buy a song from MySpace Music first you need to find a song that is for sale. You can do this by either using the MySpace Music Search, or you may be able to find songs for sale on your friend's playlists. If you just want to add songs to your MySpace profile playlist, and not buy them, you can do that too.
Start by going to MySpace Music and logging in. Then search for songs, or find songs on other people's playlists that you like. When you see the "MP3" icon you know that's a song you can buy. At the top of the playlist you may even see an icon that says "Buy Album", this will let you buy the whole album instead of just the one song.