Before you can add or edit content, you usually need to log in. If you haven't already done so, register as a user, see above (or, if applicable, request that your site administrator register you). Then hit the main page of the site you're wishing to use and look for a "User login" form. This will typically be on the left or right side of the page (it is a "block" in Drupal talk). Enter your user name and password and hit "submit".
Assuming everything's working as planned, when the new page loads it will include a new block with your user name at the top. This is the menu you use to start entering and editing content.