If so, check to see if it's Public Domain. If it is, cool. If not, you'll have to track down the publisher for a variety of rights, or to see who holds those rights.

... you have to get a license from a performing rights organization & possibly pay them a royalty. The two biggest PROs are BMI & ASCAP. There's also SEASAC, but they're much smaller. If you Google "BMI license" & "ASCAP license" their web sites explain the licensing process. A song is almost always handled by just one PRO (the artist/musician is a member of that PRO, I've been an ASCAP member for years) so you have to first figure out which PRO handles the song, then look at the licensing terms.

And, if the song is public domain (usually only old songs, long after the author has died) you can just use it as you wish with no PRO involved.

One thing to note is that the song may be public domain, but that doesn't necessarily mean you can use someone else's *recording* of the song. The recording may still be copyrighted, even though the song itself is not.