To have an RPG thread approved, you need to start in the Idea section of the forum. This section allows members to post a thread that explains a concept that the thread starter thinks would be interesting to create an RPG with. If the ideas are coherent, clear and complete, there is a fair amount of chance that the idea will be accepted. Take note, certain concepts that are too complicated can keep potential players away, though a very basic concept could leave a potentially good RPG too vague for others to want to get involved in.
Once you have been told by a moderator that your idea is good to go, you can post an Out Of Character (OOC) thread in the OOC Chat where you can have registrations, discussions about your RPG that is about to begin/has begun and whoever posts in the OOC thread must have a link to the RPG in question. Someone that is not participating in the RPG or hasn't been recruited should not post in the given thread. It is highly suggested that you PM the creator of the RPG if you want to join or have a question if not part of the RPG yet. Once your in though, it is a great place to have general fix ups and questions that every participants could all benefit from.
Now, once you have your idea approved and your OOC thread has a few interested people to start Role Playing (RPing) with you, you can post your RPG in the RPG forum. Make certain to have at the start of your first post the story and setting of your RPG as clear and nice as possible. Do not forget to post a link to the rules of this forum and write down rules which you would want to add to the RPG that you have made. As long as the rules do not go against those cited in the AO rules for RPG's, you may adapt your environment as you see fit. The more things are cleared out, the easier it will be for people to get into the story and feel the world you have created for them and yourself to live through for the next few weeks, months or even maybe years if you manage to keep the story going and players that don't dissapear on you
I hope this answered your question...I'm off to bed :P