Visual Basic - Change Machine?

I am a student at Washington State University and need help with a Change Machine...

Ok How do I make my program execute change. For example, if I have 3 text boxes with Amount of Purchase, Amount Entered, and Change Due. And then 9 check boxes with 20,10,5,1, Dollars,Dimes,Nickels, Pennies and a label box corresponding to each one, and a Calculate Button,how do I make the calculate button correspond with the label boxes corresponding to the number. For example, If I put 20 in for amount of purchase, and 30 for Amount Entered, and the Change Due should be 10. And then in there should be the number one appear in the box next to $10.

I just need help writing the beggining of the code and where at. Then I can take it from there