When you create a new Apple ID or make certain changes to your account, Apple will require you to sign in by following the link in a verification email. This is to help protect your identity and keep your account secure. Some Apple services, such as FaceTime and GameCenter, require email validation.
Simply follow the link in the verification email that says "Verify Now." Sign in with your current Apple ID and password, then click Verify Address. You can also verify by signing in at My Apple ID. You'll receive an email prompting you to verify.