As far as I'm aware, my http://www.cipscis.com/fallout/tutorials/beginners.aspx tutorial is the only general purpose scripting tutorial written specifically for Fallout 3. Is this the one you're talking about?
Changing a global variable when the player uses a consumable item is a very easy task. As the others have said, you'll want to use an effect script that is linked to your ingestible via a base effect, using a http://geck.gamesas.com/index.php/ScriptEffectStart block:
Begin ScriptEffectStart ; Do stuffEnd
Cipscis