OOC = Out of character
In every roleplay section you've got two different forums. For example you got fantasy roleplay and [Fantasy] Signups & plot discussion right under it. (Or above if you've clicked fantasy roleplay already)
In Fantasy roleplay you should only write IC posts (Ic=In character) and it should only be in a fantasy setting. Since it's not the jump in section, if you want to join a roleplay or want to make one, you should first join/make an OOC thread in the [Fantasy] Signups & plot discussion. If you look at different OOC threads you'll probably understand somewhat how things works ;)
Roleplay talk is to ask people if they would be interesting in a certain idea before you post it in the OOC group sections (Fantasy, modern, modern fantasy, horror, sci-fi, fandom, miscellaneous.) You don't have to do that, it's just if you're uncertain how people would feel about your idea and you want to check first.
Jump in is for roleplays that doesn't need sign-ups or character sheets. It's just to jump in to whichever rp you find fun. (there is an OOC section even for that forum but they usually tell you in their first post if they want you to check that out for extra information or OOC talk.)
One x one is used by people whom don't like groups (or likes groups but also wants something more personal). There you roleplay with only one partner. Also that has an OOC section to find your partner.
The libertine section is for those who wants to roleplay more sexual roleplays. Even if it's a group roleplay, you must start it in the libertine section if you know it will have a lot of sex and smutty moments. There is a teenage libertine section and an adult libertine section.
Now to your question about what to do when posting a thread. Well, that does depends on what you mean. I'm not completely sure what you get stuck on as you haven't explained it. But if you go to a random forum you can see "Post a new thread" to the right above all roleplays. You click that to post a new thread. If you want to reply to a thread you simply go down to the bottom of the thread you want to reply to and start writing your post before clicking post reply. In post new thread you must create a title to your post, you can post a link to your OOC/Signup thread IF you have one (you should create one as it's a good place to explain to people what you want from the rp, and have OOC talk, character skeletons if you want, etc. No Out of character talk is allowed in the In character thread.)
Then you click on what writing level your partners should be on, if you put your cursor above them you'll see what everything means. On content rating you should click on what possibly (and most likely) will come up in the roleplay so that people can be warned if it's for example a gory rp. Then you write your post and create thread.
If anything is unclear, please ask again ;)
