Prompt For Data Input When Other Cells Are Populated

I would like to create an spreadsheet that prompts me to input data into a cell (eg; 'C1' - 'Quantity Ordered') as soon as another cell on the same row becomes populated (eg; 'A1' - 'Item Code'). I need this function to apply for several rows (25 rows in total), so that if data is then entered into cell 'A2', I get a prompt to enter data into 'C2', all the way up to a prompt for data in cell 'C25' when cell 'A25' is populated.

As it is imperative that I never forget to populate the relevant cells; I would also like to 'enforce' the prompt - perhaps by preventing the worksheet from being saved until the relevant cells have had data entered into them, or perhaps by using some kind of form, rather than a messagebox.