Most likely you do not have the latest version of Java (Java 8) installed.
The easiest way to see what version you have installed is to go to the Java website and click the big red "Verify Java version" button.
Unfortunately this doesn't work in all browsers (at least not in mine). If it can't detect your version, and if you're on Windows, you can do the following to check your version:
Type Win+R to bring up the Run dialog
Type "javacpl -tab about" in the box and click "Okay"
A dialog should appear showing the version of Java installed on your system
Assuming one of these methods works, let me know what version you have, and I can tell more about the problem.