First make sure that you have discussions enabled within your course. You will know if you do, because you will see Discussions in the Course Tools menu.
Make sure you have builder status by selecting the build tab at the top of the screen.
Once you have builder status click on Discussions from the Course Toolsmenu.
Once in the Discussion module you can choose to Create a new topic or a New category. You can have multiple topics under one category. How you set up your topics and categories depends on your course objectives and student learning outcomes. Creating a category is very similar to creating a folder and can be thought of as such.
When you create a topic, you will be asked for a title, description and if the item is shown or hidden. Also you are able to associate the topic with a grade in the gradebook. Once the topic is created, you and your students can create messages and reply to messages within the discussion topic.