On this page: http://c-rpg.net/?page=chartitle
That's where you choose.
You always have a title if it was gained by retiring at a high level (Veteran, Master, Grandmaster, etc.) and can't turn it off. It stacks with other titles.
If you own strategus fiefs, it will list them all if you choose that as your display title.
You can also buy titles on the marketplace. They are randomly generated from a pool of possible names. They're usually like "the assassin from the east" or "the drunken defender of [player name]" or things like that.
Once you win the auction, you have to pay upkeep each month (10% of what the ending auction price was). I paid 500k for my auction so each month I pay 50k upkeep.
The ones where you can choose a player's name are fun to mess around with. I have "Grandmaster WITCHCRAFT, the eyeless hunter of [player name]" and I made an alt named "eyeballs" so now my title is "the eyeless hunter of eyeballs." I still miss my old title I lost from forgetting to renew it. "The invincible defender of [player name]." I spent a long time watching the titles on the marketplace until another good one turned up. Sometimes the random generator makes some kind of lame ones.
Also your forum title has nothing to do with your in-game title. If you have a custom one it's because you have been graced by the forum gods.
