To enter the CERN site, you should be in possession of a valid CERN access card. If you have asked for a visitor card when you registered and been selected, you will first receive an email to fill in some security information (nationality, date and place of birth).
Once you have filled the needed information, you will receive a printable access card by email, with an access code, in order to print it before your arrival.
You should present the access card to the CERN guard or to an automated portal (QR code). If you forget your access card or cannot print it, the access card can be printed directly from the CERN reception Bdg 33 or the main entrance B by entering the access code.