You can by first clicking on whatever topic board of your choosing from this page http://onceuponatime.wikia.com/wiki/Special:Forum and after it takes you to the board, at the very top is a box that says "Start a Discussion" with an empty space under it. By clicking the empty space, you can add your thread title at the grayed out area that says "What do you want to talk about?" and the larger part at the bottom is for your starting thread discussion. For adding topics, only existing character/location/item pages should be listed if it pertains to the subject. For example, you can add "Snow White" if the topic relates to her. Additionally, under the reference section of each character/location/item page, there is a button for you to use to start a topic about the page's subject matter, which will also automatically add the page as a topic for you.