The security of a Joomla site should not be overlooked simply because too little to become prey to unscrupulous hackers.
Simple rules:

1) Updating Joomla to the latest stable version from here

2) Keep all the latest add-ins you use (parts bot modules).
Very often the security problems lurking in these parts for several reasons (or abandoned projects written with little attention to security).

3) Other link contains advice on how to configure an entire server for your Joomla or Joomla, armed with good will because there so much to read. (In English) if you have difficulty with English used the google translation