Well, I don't know anything about Java in particular (my class is using C++), but as far as general logic, I would start by converting the decimal amount to cents, then divide by the value of a quarter and round that figure down to an integer. That integer becomes your number of quarters, and you then subtract that number times 25 form the total. Repeat for each coin until the remaining amount is 0. There, I did your homework.

As my instructor said, "Programming is an art, not a science."