Use Session based tracking to enable a login box when hitting this screen.

Pass variables back and forth to the google api to create your marks.

Adding a shoutbox or comments based form is just a matter of adding a textarea and including it in your form data upon submission.

Refreshing of the map could get tricky and you need to read up on the API's ability to read back to you. (Not familiar with it)

Creating tables in phpmyadmin is very easy. You need to decide what you want your fields to be.
ie: date field vs. text field that you store a formatted date in etc.

A decent starting point for this can be found here: http://php.about.com/od/learnmysql/ss/create_tables.htm