Subiectul: PL game - IRL

Hi there guys.

My name is Christian and I'm from Denmark. I was hoping for a bit of help from some of you - if possibly?
Myself, my two kids (age 10 and 12) and my brother are visiting London in the beginning of December and we would like to see the Premier League match Watford-Man. City played the 4th of December. However - I'm in doubt how to get the tickets???
May somebody be at assistants?
According to Watfords homepage, the tickets are on sale now - but it seems like you have to be registred to get the tickets - is that correct? And if yes - does it cost any money to be registred? I can't figured it out - sorry!!!