The main forum page (Forums) shows a list of categories, forums and subforums. When you click on the category you then see a list of topics (aka threads) within each category. Within each thread are individual posts from members. You have the option to start a new thread or reply/add to an existing one. (see below)
At the top of the page in the main navigation bar (on the right), you will see your name. This links to your account settings and profile area. Click on your Name to see what your profile looks like to others.
You can create a signature that will appear under your posts. For new members the signature is limited, but once you have more posts you can increase its contents and add links etc. These restrictions help to deter spammers and freeloaders.
In account details you can set some email options, upload an Avatar image that will display by your posts, add a profile banner that shows as an image at the top of your profile and supply details about yourself which will appear in your profile.
Under Privacy and Preferences, you have access to more forum settings.