When you first started up you will have had the option to fill in your details and then save it for later if you didn't want to send the info straight away. If you did this, then there should be a "Send Registration" file in the top level of your Home directory (the thing indicated by a house icon and bearing your short username). If that file isn't there, you can either:
a) go to Apple.com and register online there (the better way, IMO)
b) Open the /System/Library/CoreServices/ folder and double click the "Setup Assistant" application to launch it and this will take you through the same procedure as when you first booted up the computer.
HTH