During the initial setup of the phone you were asked to put in your login information for your Live ID. If you didn't log in with the account you wanted to use during that then you will have to reset your phone to factory defaults so that you can do the setup again and log in with the right account. Once an account is synced there is no way to log out other than doing a full reset.
Maveric 360: Do you have your account synced properly? If you do then it should be showing up. Make sure you are connected to the internet, either through a data connection or wi-fi, then go into the games section and look at your achievements. If they show up there then they should show up online and on your xbox as well. If you are sure you are synced correctly but they still aren't showing up, just give it some time. Some games take a little while to sync up sometimes.