Keep Your Servers Up-To-Date
If your fresh OS image shipped with a web server preinstalled, chances are it's out of date.
New exploits pop up all the time and good developers will regularly patch security risks, so you should always ensure that you're running the latest and greatest versions.
Check your Apache version
apt-get install apache2
You should be updating other software along your stack as well - such as PHP or MySQL if you use them.