You just need to wrap the two references within a Range:
Range
Range(ActiveSheet.Range("C24"), ActiveSheet.Range("C24").End(xlDown)).Select
Here's an approach that will work even if there are cells with content below C24:
With ActiveSheet.Range(.Range("C24"), .Range("C" & .Rows.Count)).SelectEnd With
i don't know if you want a formula for it or if any method will work but if you select the cell you want it to start on and hold ctrl+shift+down arrow it will select that cell and everything below it.