Go to a category, such as suggestions and in the corner beneath the title of the category theres a button that says Post New Thread.
If you go to the User Guide we have a detailed section about making new threads: http://forums.terraria.org/index.php?threads/community-user-guide-creating-posting-in-public-threads.70/#threadCreate
If your on a computer, just click forums and select the one you want. But here http://forums.terraria.org/index.php?forums/