Your creation holds copyright.
When the idea in your head becomes a book, a song, a poem, a painting or anything like that, the thing you created automatically holds copyright.
Copyright is attached the moment an original work is fixed in a tangible medium of expression, be it physical or digital.
You are free to use your creation any way you wish. You can create derivative works, you can sell it, you can display it, you can give it away.
If you wish to seek damages because someone stole your work, this is where registration with the US Copyright Office will serve you. This is why many artists register their work before they release it to the public.
All of my musical creations are registered with the US Copyright Office. It takes a while to receive the official copyright number for each piece, but it is worth it.
In addition, this is part of my family's legacy and my sons' estate when I die. My intellectual property can be used by them to generate income for an additional 70 years.