You need a valid Apple ID in order to download War Robots from the App Store and make in-app purchases using your Apple ID credentials.
You can create an Apple ID account: here.
You might want to add your Apple ID to Game Center in order to play co-op, save your progress, and synchronize it between your devices. To do this, open the Game Center app and enter your Apple ID login and password.
More detailed instructions are available on Apple’s website.