Well, the easiest (not necessarily best) way is to do this is to set the field properties on the Table to not allow zero length and make it required. This will cause an error message if the field is null or empty.
This would be the "Code Free" method. I suggest getting into VBA though, since use of these properties may cause heartburn down the road.
Here is a quick sample. Place this code into your Form's Before Update event:
Dim Ctrl As Control
For Each Ctrl In Me.Controls
If Ctrl.ControlType = acTextBox Or Ctrl.ControlType = acComboBox Then
If IsNull(Ctrl) = True Then
MsgBox "Whoa...Hey...You didn't fill in all the data " & _
"fields for this record. Please do so now.", _
vbExclamation, "** Fill'em Up Will Ya **"
Cancel = True
There ya go...
Self Taught In ALL Environments.....And It Shows!