New to RPG's in general
WhiteComic's quick tour hit all the site highlights.
As for mechanics...
The GM totally controls each game within the Rpol site regulations.
You first apply to a GM who is looking for players. This takes the form of a Request To Join or RTJ. The GM determines the format of the RTJ for her or his game.
Once accepted, you follow the GM's process for character generation, based on his/her interpretation (including house rules) of the system. Some games are run freeform, usually without any published rules system.
Once the game is begun, the GM posts the story, describing the setting, scenes, and non-player character actions. You post for your character, based on the GM's preferred format, as the story unfolds. Other players post for their characters.
Most GM's frown greatly on godmodding: posting for characters you don't control, whether other player characters (PCs) or nonplayer characters (NPCs).
The GM will resolve in-character actions, as well as out of character issues. The bottom line is that the GM's word is law. The GM can remove you from the game for any reason; there is no appeal.
You just keep posting until the story is done (or, too frequently, until the GM abruptly disappears or otherwise gives up the game.
That's pretty much all.
Have fun!