Registering a nickname is as simple as logging in. If the name doesn't exist already, it'll now be yours. If it does exist, you'll get an invalid password error.
Example: Your serial number has been marked as invalid. Please visit Strike Team for more information (SID: #).
If you've got a valid serial, you should reinstall the game and enter the serial. If you don't, buy a new game. Do not share your serial with other players.
Example: Invalid nickname (SID: #). Valid nicks: god xtf
Only a limited number of nicknames can be registered on a serial. You've reached this limit. Use one of the already registered nicks or delete one of them.
Example: Invalid password.
The password you've entered is not valid. If you're trying to register a new nickname this means the name has been registered already. If you've forgotten the password, reset it. You can do this by typing reset passwords in the lobby you're in.
If you're still having trouble, don't hestitate to ask for help in the help forum.