creating error messages if data is entered in the wrong way

Hi,

I'm creating a database for a charity. Donors can either be companies or private individuals. If i create a table listing all donors, not all fields will apply to individuals and similarly not all fields will apply to companies (e.g. Title, First Name and Last Name won't apply to companies, whereas Company Name won't appy to individuals). So when I put into datasheet view and enter data, how can I ensure that data isnt entered the wrong way - basically if a user is entering data for an individual donor, how can I ensure that they don't enter data into fields which are only relevant for company donors? Is there an error message I can create for this? Or do you know of a better way to do this?

By the way this is a coursework assignment and I have to enter some data in datasheet view, although I know in practice users should only be able to enter data through forms, not datasheet view of the table.

Thanks for any help in advance!