http://en.wikipedia.org/wiki/Cron
Generally by using @hourly or "0 * * * *" in your crontab file.
The "0" says "every time the minute is :00" which of course works out to hourly. But if you want to run it 30 minutes past the hour every hour you might do "30 * * * *", for example.
See the wiki article for more details.
