Piwik has been installed into /var/www/piwik/

You should point this to the DocumentRoot of your web-server:
   # ln -s ../piwik /var/www/htdocs/piwik
(make sure you use a relative symlink since Apache is chrooted)

You will also need to add these lines to you php.ini file:
    extension=pdo.so
    extension=pdo_mysql.so
    extensions=gd.so

and proceed to complete the installation by reading the online
documentation. Which can be found at:
http://piwik.org/docs/installation/

You can ensure you have a working install by accessing:
http://<localhost>/piwik/
