The Core Rules say: “Natural 20 is automatically a critical doing maximum damage.”
But: Players love fantastic stuff to happen when they roll a natural 20 in combat. Various variants suggest themselves. This is what I like to do:

Print the Critical Hit Tables from the HARP Lite manual (get it from the HARP site in the free download section) around page 56. When players roll a natural 20, roll 1d100 on the appropriate critical hits table. Add 5x STR bonus (or DEX bonus if using a light weapon and prefering DEX) to the result.

Example: A thief with DEX 16 using his knife and rolling a natural 20 gets a 1d100+15 on the puncture critical table. (I suggest you don’t use this table against players…)

Last session, the level 1 priest decided to walk out into the forest in the middle of the night, trying to talk to the trees. A Kappa (4 HD) surprises him. He flees back to the village. The kappa follows. The priest draws his bow, takes aim, and rolls a natural 20. I roll 1d100+15 because the priest has DEX 16. The result is an awesome 92+15=107!

“You impale foe in the lung—he’ll die in 12 gurgling rounds. Foe takes 29 Hits, is stunned 12 rounds, bleeds 3 per round, and is at -20.”

I’m sure my players will remember this event for a while to come!

Let them roll a second attack. Repeat as long as a natural 20 comes up. This is simpler, but lacks the gory details.