Creating test cases in Java?

Hi, I was giving a program to sort numbers. It's been done using arrays, but what we need to do is to create test cases using j-unit(eclipse). To be honest with you, I'm a little lost on how to start or what to write. We have to create as many cases as we consider necessary. For example, we could expect positive, negative, decimals, or 0 numbers. So, we need to make sure that the sorting algorithm will work with those types of numbers and we won't get any error. Any help will be greatly appreciated. Thank you