The specific law will depend on where you are based, but if you are able to prove that the work is all your own that should be enough for you to assert your intellectual property rights. Include some text in a readme file which states it's copyrighted and cannot be sold.
It doesn't have to apply to the whole game. If someone took the music, a graphic, or even something in the gameplay which is unique to your game, they can be covered by copyright.
You will need to be able to prove it came from your game e.g. you should make a timestamped backup made with a system which can't have the date and time artificially set. Uploading it to Aminet would be a good way to do this because all the files have an upload date.
