so you install Redmine and apache2 is giving your grief. You probably tried to use the default passenger. if
But... if you can't go to your redmine directory and run:
$ ruby script/rails server webrick -e production
then browse http://yourserverip:3000
This article will not help you
This assumes you have a working Redmine with ruby


First off you will want to disable the apache passenger:
$ a2dismod passenger


You will want to run the script that builds the passenger:
you should be using rvm if your not then owell.
I went to my current build wrapper:
$cd /usr/local/rvm/wrappers/ruby-1.9.3-p429
then I ran:
$ ./passenger-install-apache2-module
I was missing a bunch of stuff so I apt-get all the stuff missing and run it again
you should see it building.

it will then tell you to modify apache config:
$ nano /etc/apache2/apache2.conf I added to the top of the config:

LoadModule passenger_module /usr/local/rvm/gems/ruby-1.9.3-p429/gems/passenger-4.0.2/libout/apache2/mod_passenger.so
PassengerRoot /usr/local/rvm/gems/ruby-1.9.3-p429/gems/passenger-4.0.2
PassengerDefaultRuby /usr/local/rvm/wrappers/ruby-1.9.3-p429/ruby

Now save the file
next edit your default site:
$ nano /etc/apache2/sites-available/default


<VirtualHost *:80>
   ServerName www.yourhost.com
   # !!! Be sure to point DocumentRoot to 'public'!
   DocumentRoot /somewhere/public
   <Directory /somewhere/public>
      # This relaxes Apache security settings.
      AllowOverride all
      # MultiViews must be turned off.
   Options -MultiViews
   </Directory>
</VirtualHost>

restart apache:
$ service apache2 restart