Before you can add or edit content, you 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 and look for the "User login" form in the left 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.