There's no way to change cities right now, but you don't need to in order to join the various guilds. All you need to do is level up your class to level 20. As you are 14, you have a little ways to go. After that you can go to the guild and do your first class quest. After that quest, you are then considered a part of that guild. Right now though there aren't any benefits or perks associated with them. In the past, we used to get guild marks that we could trade in for skills and abilities, but this system was suspended some time ago until the devs decide how they want to re-implement it. So for the time being, the only thing you get out of it are some story quests.
I should also add that you aren't limited to only one guild. You can do the story quests for every class in every guild, it's really just a story mechanism at this point and has no real bearing on gameplay.
Edited, Jan 2nd 2012 12:13am by Lantesh
Variety is the spice of life. Except on Dune, where it's worm poo.