In order to play X-Wing Vs. TIE Fighter online, you need to consider the following steps.
To play online you need a separate multiplayer lobby. The Microsoft Internet Gaming Zone (IGZ) has quit support for XvT/XWA in 200X. After that, the community created it's own client, the Errant Venture. However since Voobly, a fan-made, commercial endeavor which resembles IGZ, has added support for XvT, XWA and even BoP, that's the new client to go. Some people still use gameranger, but the activity there is very low.
After signing up there, you download the client and use the Links on the website to join the game-specific lobbies, for example the XvT lobby.
To go around that voobly.com visit each time in your browser, you can bookmark the link “join the lobby” of each game. Having done that, in Firefox and Opera you can go to Manage Bookmarks and drag-and-drop those bookmarks to your desktop. This creates desktop shortcut for this lobby. This is way more convenient and faster than joining the lobby over the Voobly website.
Voobly has gold wine support, this means Voobly works fully under Linux using the Wine compatibility layer.