Ellie
Here's one of many ways. Open the form's property dialog box, then select the BeforeUpdate property, click the ... button and select code builder. This will allow you to write some code that is executed before the recorded is committed. Now you can check the fields to make sure there are data in them prior to saving the record. For instance if you wanted to ensure that the the Last Name field was filled out the code would look something like this.
Private Sub Form_BeforeUpdate(Cancel As Integer)
If IsNull([LastName]) Then
MsgBox "Some message", 0, "Some Title For MsgBox"
End If
End Sub
you can check as many fields as you like. Substitute your field names inside the brackets [AnyFieldName]. If you want to prompt the user for the specific field not updated then use a separate if statement. If you want to have a generic message (i.e. Must fill out record completely) then you can use 1 If statement with Or i.e.
If IsNull([LastName]) Or IsNull([FirstName]) Then
MsgBox "You must enter a last and first Name", 0, "Unable to Save Record"
End If
HTH,
JC