Is this for a homework assignment? If so, you probably don't want to use Garret's solution since you will miss the whole reason for having this assignment: learning Java!
With that said, how do YOU turn a numerical representation of a number into words? For example, what words do you use to represent 123456? How did you come up with that? You should come up with other examples. Perhaps it is better to start with smaller examples and build up. For now, don't worry about Java. Try to describe the steps you use in your native language. Of coruse, if you want to post your ideas here, you should probably do it in English, but any natural language will work.
Let us know what you come up with and we will be glad to continue to help you.