Enter code to use app

Hi I'm developing an android app and I want my app to ask for a keycode(word or numbers) to enable it's usage, this is because I want to distribute this keycodes myself, but I want the keycodes to work once in a device, I suppose I need to check the entered keycode with some other in a server,

So, which tools can I use?

I've been reading about google app engine but I don't know if is the way to go. Thanks