In order to post a message to a category, you will first need to have access to the category. If you have access and there isn't a topic listed already, you can create one by clicking the blue Add New Topic button from the top of the page. Once you click the button and the form appears, make sure that you select the appropriate category that you would like to post the new topic (and message) to. Type your message and make any other selections as needed and then click the Create Topic button to post your message.