Aion is currently free to play in Europe. The North American version will launch this Spring. To sign up for Aion EU free to play, if you haven't already, go to aionfreetoplay.com, make an account by clicking log in, download the client and you're in.
EU eng Server: Spatalos
Characters: Wifeofsylar and Malicemizergackt (I dunno which will be my main.)
NA Server: Vaizel
Visit aiononline.com for info on when they will switch over. Currently there is a reactivation event going on.