First and foremost- calm down. No one has ever overdosed on cannabis, and you're not about to be the exception.
Take a nap- sleeping it off is a safe first resort.
Hydrate- chances are you've got a bad case of cottonmouth right now, a common side-effect. This can also dilute the contents in your stomach.
Eat- Try eating something non-medicated to alleviate some of the symptoms.
Peppercorns- Try chewing 2 or 3 peppercorns if you're feeling too high. Cannabis and peppercorns have some of the same terpene profiles and can counteract the extreme effects of eating too much.
CBD- if you've got any CBD around, this can give some patients almost immediate relief.