Mandatory fields

Hi everyone,
can someone tell me how can I create mandatory fields in Excell?

Thanks.

Ok, I will try to be more specific. I am creating an Excell database to monitor the showroom traffic in some motorcycle dealers. The salesmen must enter some data to give me usable feedback. In one case (for example), they must choose if the bike they are trying to sale is NEW or USED and I want to make this entry mandatory in order to proceed to next cell (CUSTOMERS NAME). I tried to use some data validation using another sale as refference (if the total number of new and used bikes is different from the total number of enquiries, I wanted a warning sign or something but in this case the cell value changes automatically -function- and the data validation only works when entering data manually...)

Thank you again!