Ciao, i am a newcomer...
Welcome, Salvo.
You need to vote (there should be a button or something at the top of the page) to register for this series. Then, after your registration is confirmed, the password for the race will appear in the specific race thread (in the "UKGPL Races" forum) 24 hours before the race time.
The practice server (if/when there is one) will probably use "60fpsV1" as a password. You should really try to join the practice server to make sure that your system is set up properly before the actual race happens.
Good luck!