If you require a room at the CERN hostel please contact them directly via cern.hostel@cern.ch or +41 22 7674481. You will need to supply your credit card details to secure any booking. If you are a registered CERN User and have a CERN computer account you can book a room online via http://smb-dep.web.cern.ch/en/CERN_Housing .
If the CERN hostel is full you can find a list of local hotels that have preferential rates for CERN via http://smb-dep.web.cern.ch/en/hotels-rates .