You pick one of Scissors, Rock and Paper. At the same time (trust me, no cheating) the computer picks one too.
The winner is decided on basis of the following 3 rules.
To make your result reasonably significant you should play at least 100
times.
If the applet does not start, you should update your Java VM. All you need is Java Runtime Edition from http://java.sun.com/j2se/1.4.1/download.html.