Creating a macro for displaying a pop up box

Hi,

I've created a spreadsheet for keeping a selection of data. Once a cell reaches the number 10 I'd like a pop up box to appear saying "Jon now has a one match ban", then once the number I input reaches 20, I'd like it to repeat the pop up message, when the number is on 11, 12 etc then I'd like nothing to happen. How do I go about it?

Thanks,

Steve