Logging In
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. Then hit the front page of the site and look for a "User login" form on the left hand column. 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.