Empty Fields - Required Fields - Field Rules

I would like to enforce the entry on particular fields in products, custom or not.
And a trigger that would not allow the creation or update of a record if some fields are left empty.

This is because we will hire someone to do the data entry and we would like to make sure
he won’t forget or skip something.

Or again having someone on the weight or price field using a comma instead of a dot for decimals.