Cron

First make sure cron is properly configured.  Check /usr/lib/cron/cron.allow and /usr/lib/cron/cron.deny.

In Ubuntu none will exist by default so if you want to allow everybody to run cron then

# cd /usr/lib
# mkdir cron
# touch cron.deny

This creates and empty cron.deny allowing all users to run cron.

To edit a crontab through the command line, type:

# crontab -e

Add a line for you site replacing example.com with the name of your site

45 * * * * /usr/bin/wget -O - -q -t 1 http://www.example.com/cron.php

References: Configuring cron jobs - Crontab quick reference - Ubuntu CronHowto  
Improving the performance and flexibility of Drupal's cron by using the Elysia cron module