This is a usefull Post that i found in http://www.joomlasecurity.info
Joomla Security Info is your guide how to keep your Joomla-based website secure. We assure you that the developers of Joomla! CMS take security threats seriously and Joomla is in fact very secure. However, you need to understand that there is no such thing as a 100 % secure CMS. To keep your Joomla website secure from hackers you need to take the following steps.
* Use the latest Joomla security update
* Use only secure third party plugins and keep them updated
* Use secure username and password for administrators
* Use an SEF component that makes your Joomla more secure
* Use a secure web host / secure server configuration
* Don’t tell everyone about your configuration
* Write-protect your Joomla configuration file (make unwriteable)
* Delete Joomla templates that you do not use
The tutorial in ddep can be found here:
http://www.joomlasecurity.info/