Welcome to the www.planewalker.com roleplaying board! We're glad to see you here and hope you'll find plenty to entertain you. This FAQ is designed to provide a few bits of advice or notes so you can settle in quickly here.
1) What are these sticky threads up here at the top?
When we were deciding how we wanted to run this forum, we wanted to support two different forms of roleplaying.
The first format that we wanted to support was custom gaming. These are threads that are started by a particular GM, and generally run under that GMs own rules. This is up to whoever wants to run one to start, no permission is needed at *all*. Just toss out a thread and recruit some players. GMs are free to ru the thread as they wish, and may require full character sheets, or certain levels of play, may be open to new players, or closed as a campaign underway. To join these it's usually polite to ask the GM for the thread if you may join up.
(Highlighted because folks kept missing it.)
The second exists within a shared world - run by our moderation staff - these threads are open to all, and are a great place to get your feet wet. This is a shared world, and will have less large changes from standard canon, simply an effect of being open to all. Anyone can just stroll into these threads, and they are usually associated with a particular location. The first post of each of these threads will have the place description and links to any previous threads in that location as well. Gaming in these threads will be more of a freeform style - but we would like if you'd post up a character description for everyone to see. It's a good way of introducing yourself.
RECENT CHANGES: Check This Post
2) Some general advice to GMs on running game threads
Obviously keep posting and the game will stay alive. This goes for players as well as GMs. Beyond that it's entirely up to you how exacting you want to be when you start a thread. You can ask for stats and dice rolls if you want, or just stick to free form play.
3) Character description? Where would I put that?
The character description thread should be one of the sticky threads at the top of this board. The first post of the thread will give suggestions on writing up your profile, as well as links to any previous character threads. You should be able to link back to these posts in your signature file so everyone can see who you are. Iconic characters should put up a profile too, we may have newbies who don't know who you are. 
4) Where can I find information on Planar Cant?
As Lok found: a Dictionary
Also, cant is remarkably simular to certian aspects of cockney in london, so you can use that as a model easily as well.
5)I assume that whenever plots makes it possible we can just move our characters about to the various threads created by the admin?
Right.
6) Also I assume if I want to stat a roleplaying thread I can just do that or should I get it approved by someone?
Just start one up, no need to ask permission at all. The admin threads are there to give a 'touchstone' if someone's completely new to Planescape or a character is between adventures. They're intended as start points - not as the only threads on the board. If you want to start your own thread, either spinning it off the main, or just recruiting for a game to run for yourself - go for it.
7) So can I start a freeform thread of my own here?
Yes.
8 ) Is there any place I find a definite map of Sigil? Or even a guide, there just needs to be something that everyone on this forum will accept as Sigil, so all the taverns or other important places will be in the same place for everyone.
Bob found us a good one at: Sigil Map
So - from here on, ask questions and we'll do our best to answer them here. 
Where can I find information on Planar Cant?
As Lok found: a Dictionary
Also, cant is remarkably simular to certina aspects of cockney in london, so you can use that as a model easily as well.