Distant Star on 20 December 2009
You cannot patent an "idea". Your best bet is to write an Ebook/Comic book. You'll have a copyright protection. If it's good you might be able to license it. This will only work if your idea is storybased.
If your idea is a gameplay mechanic make a mod and eventually game. Go to moddb and find people who want to put effort in it.
Share your idea. I don't believe someone will steal your idea. In the end you are the masterbrain that came up with it. So people/modders would like to have you in their team.
Hopefully it isn't a "GTA with everything" idea, because everyone came up with that already.
