how to create proper VBA to delete cell contents?

I am trying to get my spread sheet to delete contents in cell B24 when a number of 15 or more is enter in B22.
B22 is a minimum quantity of product we need to order to not pay a drop charge. 15 or more product and there is no drop charge fee which is entered in B24.

I have tried various VBA's I found on the internet but I can not seem to make them work. My spreadsheet is saved as a Excel Macro-Enabled Workbook (*.xlsm) .

Is there a specific VBA that would work for what I am trying to achieve?