As a new user, your first 5 posts/threads need to be approved by a forum moderator before they are visible, these are normally reviewed in the first 48hs. Once you pass this threshold you'll be able to post freely without intervention in the forums in most categories.
Buy & Sell threads will always require moderator approval before being visible to the rest of the forum user base. You will gain access to this section once you reach the 25 post mark.