Securing your Joomla! powered website isn't something you should be thinking of, its something you need to DO. Unfortunately there are to many hackers out there that target joomla sites and take them down and in some cases they delete your data. First and foremost you should have a back up solution setup on your site. I personally like and use a plug-in called Joomlapack every site I develop gets that installed on it. This tip is NOT for Joomla! developers this is for end user and web site owners who are not developers. If you are a developer and are looking for detailed information on site security please look here

As a web site owner there are a few things you can do:

1. set a good strong password for your administrator account (it also would be good to NOT use administrator as the user name)
2. keep your site updated. Joomla! has a team that is constantly patching and fixing security flaws and release a new update as often as they can.
3. keep any installed extensions updated as well. some extensions will let you know if there is a update available but for most you will have to visit their site to check on updates.
4. Enabling SEF url's will also help. with SEF url's not only does it help you with search but it also hides certain information about your Joomla! install.