You put the -1/-1 counter on Barrenton Medic as a cost. That means it happens when you activate the ability, not when it resolves. If paying the cost causes the creature to have 0 toughness, it’s put into the graveyard before you can untap it and before you can even pay the cost again.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.