Really need help with excel

Hello,

I haven't done any coding in years, and even then i wasn't really that good.

Basically i have two sheets within my spread sheet, named Done and TBD.
In each sheet starting from the cell A2 i have a list going down of customer numbers.

What i am looking for is some sort of code, or guidence so that if any of the customer numbers in the TBD sheet are not on the Done sheet, those customers numbers are copied to a 3rd sheet named "Left" and then at the side of that data (coulmn B) a total number of the customer numbers that were copied to the 3rd sheet is shown.

maybe not possible, but i also have dates etc for events, so the customer numbers can be organised by month, would it be possible to get excel to work out what is left to be done from the 3rd sheet for each month?

thanks for your time, and hope someone gets what i'm going on about.