There are many fine guilds on Cenarion Circle for you to choose from! Which one you choose should depend on what your character is like.
I'm not terribly familiar with what guilds are active hordeside, but some of the more active alliance guilds are:
In alphabetical order!
AAMS: Neutral guild with branches in each faction, specializes in cross-faction delivery and translation services.
Contact Derscha (alliance) or Kezrin (horde)
Conclave: Seekers of knowledge, and just about anything else! They are one of the most inclusive RP guilds I've heard of, not many characters are a bad fit for their theme.
Contact Genevra
Crimson Blades: Mercenary type guild, they are active in-game, and also very prolific forum RPers!
Contact Adrazel
Feathers of Iron: Mercenary type guild, they also host tavern night in Booty Bay every Tuesday night.
Contact Franziskah
Pia Presidium: A holy order, they are the quintessential lawful good paladin type guild, complete with knighting ceremony. They are currently going through a change in management, but they are still hosting troubadour night and clinic night on alternating Mondays.
Contact Zephilyn or Freediction
Stormwind Union: *attempts not to gush too much* Law enforcement guild, accepting all roles of law enforcement, including patrol officers, coroners, lawyers, K9 officers, forensic technicians...and just about anything else (we have a regular citizen rank). Lawful good, of course.
Contact: Orwyn (or I'll be happy to tell you more if you poke me in-game.)
Terra Incognita: Mercenary type guild, does what needs to be done for the good of Azeroth. Mostly ignores conflict between alliance and horde.
Contact: Kordrion or Ketlan
Hordeside guilds... uh...
I only really know of AAMS, Da Doctas, and Doomguard. And Modas, if you're into being evil.
The most fantastic thing about this server is that you are not limited to interacting with only the members of your guild, our community is very tight knit, and in many ways it's like the entire roleplaying community is its own big guild, and the RP guilds themselves are just smaller, more specialized cogs in the machine :)
Good luck!
If you grab me in-game, I will be happy to tell you more about any of the Alliance guilds :)