how can I copyright my game

I am working on a game and want to put it on steam as alpha but I’m hearing that if I put my game out there without a copyright, people can copy it. does anyone know how to get one or where to get one? Does the copy write protect just the game or protect all that goes with it. such as images and sounds. or do I need to have separate CW for each of the images and sounds?

found this , i hope it helps

Good Luck