Redmine installation on Ubuntu ... now what?

Unfortunately the redmine Ubuntu package requires some manual configuration before you can get started. Now that redmine is installed, you still have to make it accessible through a real web server. An easy way to do this is with Apache with the mod_passenger plugin.

Just edit your /etc/apache2/sites-available/default (or default-ssl), adding:

RailsBaseURI /redmine
PassengerDefaultUser www-data

Then configure /etc/redmine/default/database.yml as necessary and run ln -s /usr/share/redmine/public /var/www/redmine. That should do the trick.

You can read more at http://www.redmine.org/wiki/1/HowTo_Install_Redmine_in_Ubuntu.