Are you looking for the legal or illegal options?
If you want to go down the illegal route, download a program called Limewire from the link below. Click on the get basic option and download the free version of limewire.When the download is complete, install the program.
A Limewire icon will appear on your desktop.Double click on the icon and Limewire will open.On the left hand pane you should see some options such as audio,images,video, etc.click on the audio option, and then type in the name of the song that you want.After a couple of minutes lots of results should appear. now all you do is double click on the song you want, and it will start downloading. One tip to remember is to pick the song with the highest number that appears on the left of the list of results. This should mean that more people have the song available for you to download from, and should result in faster download speeds.
http://www.limewire.com/download/