Copyright your song if it is not yet published. OGSR has found the “Copyright Resources” on this page to be helpful.
If your song is not copyrighted and you don’t plan to secure a copyright, you still MUST fill out the OGSR Hold Harmless Waiver, and put the following on the “Fill in copyright/registration information” lines: I wrote this song by myself and am solely responsible for its music and lyric content. I realize that I am not protected by copyright and I am willing to take that risk.
Here are two CD Baby articles on copyright your songs:
US Copyright Registration Portal: