Activated charcoal while very good at absorbing and eliminating many poisons isn't something you generally administer at home, usually just by the vet.
Here's a link that explains it uses.
http://www.emedicinehealth.com/activ...article_em.htm
If would be good to have hydrogen peroxide around though and you can give 1/2 tsp to induce vomiting to get rid of the toxic substance that way. This must be used within an hour to an hour and a half to be useful. Dogs are like humans though you don't always want to induce vomiting - like in one instance if the dog drank gasoline or some other caustic substance that would burn the stomach or esophagus - you wouldn't want to induce vomiting and have it go through the esophagus again.