Download and install the Phoenix Worx client. If you have already registered at the forums, you can use the same user info to login to the client, otherwise you will need to register. Once you've logged in, you can either create a game, or join an already existing one (using the "Create" and "Join" buttons respectively).
Note that you need to forward a set of ports from your router to be able to play TA online. For more information on this, read the thread titled "Router config/Timed Out error" in the Technical Support forum.
Also note that any questions and problems about playing TA online, or the client, belong in the mentioned Technical Support forum.