I'm afraid you can't cancel a Ryanair flight, only change them.
Click here for the link to the Ryanair webpage. Any charges for the amendments you make need to be pay directly to Ryanair.
You will need the 'My bookings' section at the top on the Ryanair webpage.
You will also need your airline reference (NOT your Thomas Cook reference) and YOUR email address used at the time of booking your holiday. These are both found on your invoice. The airline reference is in the Flights section shown below).
Should Ryanair make any changes, they'll email you themselves. Just make sure you let us know about any changes, too, so we can check the rest of your holiday isn't affected, such as your transfer or hotel reservation. This includes any changes you wish to make yourself as well as those from the airline.