Select a forum and post a topic within it with a title. Like this one.
You can select a forum from the drop down list above or by clicking on the heading or reply from another user.
Users can then reply directly to that topic or privately to the member who posted it.
You can search for members and message them directly from their user profile page. You must follow them fist by clicking on their profile.
When you post something yourself or find a topic you are interested in, don't forget to select the subscribe to topic box to be alerted of new posts on that topic.