Access Tip -  Require that specific fields contain data

Although it's usually okay for some fields in your tables to be blank, almost every application has fields that absolutely must be filled in. If missing data is causing headaches, you can take steps to prevent incomplete records from being created. Setting a field's Required property at the table level prevents users from being able to save a record if a required field is Null. 

 Open the table in Design view, select the appropriate field, and set its Required property to Yes. If your table already contains data, Access prompts you to check whether existing records violate the new setting when you save the table.  If you click No, the existing Null values are accepted as is.  However, new records must adhere to the required entry rule.