Become The Active Cell Than The Userform Is To Open Up

I have created a user form that has 6 input boxs and at the bottom of the userform has a seventh box that has a running total. I have two buttons one to "accept" and one to "cancel". My prloblem is creating the code to make it work correctly.

1) if a cell in B5:B20 become the active cell than the userform is to open up.
2) once the useform is open the user can input up to six different numbers into the input box's which will show the runinng total in the seventh box.
3) Once all numbers are inputed into the input box's the user can hit "accept" to post the total into the active cell or hit "cancel" and nothing have nothing happen.