The role playing part of the forum is divided into four different forums: water tribes, earth kingdom, fire nation, and air temples.
When posting, make sure that you are in the correct nation. The reason the forum is divided, is to make it so that new members to the site can find a topic and know which nation it is in.
When creating a topic, simply pick which nation you want to post in, then create a topic and start posting.
The only time you need to ever worry about creating a new topic is if you leave the nation you are currently in.