Go to http://www.javatester.org/version.html and check your version.  The minimum requirement is Java version 1.5.1.
Note: Mac users should use the apple software update feature to install Java.
PC users can get the latest Java version from http://www.java.com

An alternative way for testing whether you have Java is to go to http://www.java.com and scroll down the page until you see "Do I have Java?"...click there
If you do have Java, test that it's working by clicking on the "Congratulations!" screen on the Java site