I had same problems. They design site to be confusing when you want to cancel.
On the cancel subscripition page it says "you can find out how to cancel your pass by clicking 'continue' below"
and guess what clicking does nothing!
@Houli if clicking My Account takes you to the homepage, you've logged in with the wrong username. It's possible to have multiple SkyIDs and only the one attached to NOW TV will work. If you've had Sky Broadband or TV in the past your browser may have auto completed your old username. Try other usernames you might use or the forgotten username link on the login screen.
How do you cancel a NOW TV
To cancel now TV go to your my account page on a PC (You will need to be logged in to do this.) and click the cancel pass button on the appropriate pass. (The different pass types must be cancelled separately) and follow the dialogues that appear after you have finished the consolation process. If you go back to your my account page you will see a notification indicating your pass has been cancelled