Glad to see you found your way here. I'll try to answer your questions :)
How often you're expected to post differs much from every RP. Some RPs move one page or more every day, while many only gets one or two posts a day. There are even RPs who only get a few posts per week but are still considered active. It depends mainly on what kind of RP it is; chapter based RPs generally move slower, since the players often want to type up longer, good looking posts. There are RPs where players only post a paragraph at a time though (four lines is absolute minimum, or else it'll get pretty spammish and you should probably go RP over MSN or something if you really want to make posts shorter than that).
As for your last question, it also depends on the RP. The Game Master (starter of the thread usually) sets up rules that most often contain explanations on how you're supposed to go about playing the game. Some RPs are mostly talking and interacting, while some will require you to almost write a fanfic of your own, only within the limits set up by the specific rules.
Hm, conclusively: you can find just about anything here, and it's better that you ask the GM of a certain RP you're interested in :3 A good idea is to ask in the corresponding OOC thread (in the Roleplay Lounge subforum) or VM (visit messaging) the Game Master directly.
