Go to google.com.
Type the name of the song you're looking for into the search bar in quotations followed by "MP3." Then click the "Search" button. For example, "A Hard Days Night" MP3. If you're looking for a song that has been recorded by more than one artist, include the artist's name in your search to get more specific results.
Enter a partial song title in quotes followed by the artist's name. Sometimes you might be trying to find a song that you don't know the full name of. If you search for a couple words in the title along with the band name, Google will usually find what you're looking for. For example, you are trying to find "Everybody's Got Something To Hide Except For Me And My Monkey" by the Beatles, but you don't know the full name of the song. You could search for "My Monkey" + Beatles.
You might occasionally want to search for a song but don't know who wrote it or what the title is. In this case enter a line of lyrics from the song in quotes. If you search Google with partial lyrics, it will likely find a title and artist. For example, searching for "all my troubles seem so far away" would return results for "Yesterday" by The Beatles.
Click on any of the search results that appear. Search results will be ordered by the most to the least relevant, although sometimes the best result may not be the first one.