Find out the song title, artist, and album. Type a lyric from the song into your preferred search engine if you don't have any of this information, or reference an old CD, cassette, or record.
Enter the song title and artist with the word "listen" into your search engine. Put quotation marks around the song title or artist name if they contain common words arranged in an unusual way, to be certain you get relevant results.
Review the results, and click on those that contain both the artist and the song title.
If you don't get good results through your search, visit sites such as YouTube, Grooveshark, Pandora, Last.fm, or Jango, among others.
Repeat your search on each of these sites according to their protocols, entering the artist and song title as needed.
If you still don't find the song you're looking for, try iTunes, Amazon, or another reputable mp3 vendor.