Q: Game center

Hello iPoan!

I would certainly want only those authorized to be using my Apple ID or Game Center credentials.  

If someone else is using your information, you can check out the steps here: If you think your Apple ID has been compromised - Apple Support

Using the Apple ID website to reset your password is probably going to be the quickest way to prevent another person from using your Apple ID/Game Center as it will prompt for the new password on all your devices with your credentials signed in. 

Best wishes!