Primary RP cities are Cove, Trinsic, Vesper and Yew. Each city generally has a couple of guilds around it, with several guilds not strictly based around a particular city.
Every city/guild is different, so you can pick whatever one fits what you want best.
Off the top of my head, there are:
Cove - Baronship of Cove
Trinsic - Duchy of Trinsic, The Elven Guild
Vesper - Vesper Militia, Vesper Trading company, Stirling & Kendall Trading Company
Yew - Guardsmen Militia
General - Flohlen D'lolth (based in Malas), People of Sosaria (generally used to get characters into the war ring, without aligning themselves to a "faction"), Brigands of the Fort.
There are also a bunch of guilds I'm not sure about the activity of. Citizens of Skara Brae, who are a largely French guild, and often travel to other cities to run markets. One based in Britain, who's name is evading my mind (abbreviation LB).
Plus, I'm bound to have missed a load.
Honestly, I think it's best if you post what you want from RP.
For example if you want to play an elf, then Yew's not for you.
Or, you can just mooch around and RP as a blue (or try and get into P-S), and see what suits you and your character best.