To setup Piwik navigate to:
http://${VHOST_HOSTNAME}/

Install and upgrade instructions can be found here:
http://piwik.org/docs/installation-optimization/

For general information please read:
http://piwik.org/docs/

To setup a MySQL database modify the following SQL:
CREATE USER 'piwik'@'%' IDENTIFIED BY  'very_secret_password';

GRANT USAGE ON * . * TO  'piwik'@'%' IDENTIFIED BY  'very_secret_password' WITH
MAX_QUERIES_PER_HOUR 0 MAX_CONNECTIONS_PER_HOUR 0 MAX_UPDATES_PER_HOUR 0
MAX_USER_CONNECTIONS 0 ;

CREATE DATABASE IF NOT EXISTS  `piwik` ;

GRANT ALL PRIVILEGES ON  `piwik` . * TO  'piwik'@'%';

You might want to retighten permissions on the config/ directory once you have
completed the setup.