http://z14.invisionfree.com/12th_Cen...wtopic=13&st=0

the first post on this thread explains which factions are taken and available... just post in the thread which faction you wanna play as, the moderators will say yes/no and then once you've got a faction post in its relative sub-forum (there's a map somewhere but i can never find it) a thread about your faction and its assets/troops etc...

hope this helps