To use tha javadoc utility all you have to do is prepare your code as you would any, and during compilation time instead of writing:
javac ScrollingBanner.java you write
javadoc ScrollingBanner.java on the command prompt.
Lot of information will be shown on your command prompt of your screen describing everything that has been done. You can read that but most importantly go to where you saved your ScrollingBanner.java file and locate ScrollingBanner.html. Open that file and that will be what is needed for the assignment.
Hope i came to your help and sorry fro my long story.