Legal Question in Intellectual Property in Michigan

It is legally yours, and copyright exists in the song as soon as you have finished it. You should register the copyright in it, with the US Copyright Office. You can do this online at the Copyright Office website. Your lyrics/music can always be infringed (stolen) -- just like any other property -- but registration is needed if you intend to sue for that infringement, and in order to collect the highest amount of damages and attorney's fees.

Whether you register your copyright or not, place a copyright notice on or near the lyrics, wherever you are posting them. Include the word "Copyright" (or the encircled C symbol) your name, and the year the song was created. This serves as public notice of your ownership.