How do I copyright my idea for a videogame?

You can't copyright the idea, but you can protect the software and graphics with copyright law.

However, copyright doesn't prevent others from "doing" the same thing; it only prevents them from copying your work or making "derivative works" without permission.

The best protection for a valuable new game is to file for a patent on its unique inventive processes, and a patent (if issued) will prevent anyone from "doing" the same thing in your country unless they get your permission (or buy your patent). Filing a "provisional patent" is usually quite quick and and inexpensive. The whole process, however, may cost you thousands of dollars. Copyright is "free", but has limited protection.

You can't copyright an idea. But you can copyright the game once you write it.

You cannot copyright and idea, you can however copyright the game.

You need to develop the game then seek a copyright.