If you haven't already got a first class ticket, you will need to buy, then activate it. To do this go to the 'Profile' section at the top of the page and then into the tickets section. If you have already bought a ticket, then all you need to do is click on 'Activate'. Please allow up to 24 hours from doing so though.
If you haven't bought a ticket, go to the online shop and buy whatever ticket you like.