select range excel macro

I can build a MicroSoft Excel 2000 macro to perform a function on a single cell that I select/highlight , on a column that I define or on a range of cells that I predefine in the macro. How can I write a macro so that it is dynamic enough to only run on a range of cells that I highlight with my mouse. I only want it to be performed on the range that I highlight / select, each time it may be a different range or worksheet.