You can start in the astral astoria immediately RPing, and other people might join in on your RP.
You can join a group whenever you want by going to their board and applying in their recruitment forum. Some groups are more selective than others so don't expect that just because you apply you will be admitted.
You may also begin posting in the battle grounds whenever you want, it is merely recommended that you start out in the astral astoria to get the feel for things. The Battlegrounds are open for any kind of RP in any setting you want, while the Astoria is designed to let you get some practice in a pre-determined setting.
Have fun, and remember the most important rule: be realistic!
Welcome to TRF.
Edit:
You can claim yourself to be a Sith apprentice, but to move up in ranks and gain more power you must find a master and go through training. To do this you could either start an RP thread in the battlegrounds or go to the board of a Sith faction (such as TNSO) and post in their forums either IC (in character) or OOC (out of character) and try to find yourself a master, or at least gain admittance to the faction.
Oh, and [url="http://pub5.ezboard.com/bthegungancouncil"]http://pub5.ezboard.com/bthegungancouncil[/url]
