According to the 2007 EM model of Clinical Practice of Emergency Medicine, the following topics within Toxicology are considered the baseline expected knowledge of Emergency Physicians / Residents.
Analgesics (acetaminophen, NSAIDs, Opiates, Salicylates), Alcohols (ethanol, ethylene glycol, isopropanol, methanol), Anesthetics, Anticholinergics & cholinergics, anticoagulants, anticonvulsants, antidepressants, antiparkinsonism drugs, antihistamines & antiemetics, antipsychotics, bronchodilators, carbon monoxide, cardiovascular drugs (beta blockers and calcium channel blockers, digoxin, antihypertensives), caustic ingestions (alkali & acids), cocaine, cyanide, hydrogen sulfide, hallucinogens, heavy metals, herbicides, insecticides, rodenticides, household chemicals / cleaners, steroids, hydrocarbons, hypoglycemics / insulin, iron, isoniazid, marine envenomations, methemoglobinemia, mushrooms, neuroleptics, organophosphates, sedative hypnotic drugs, stimulants & sympathomimmetics, strychnine, and lithium.
With that said, as a medical student, start with opiates, cholinergics & anticholinergics, acetaminophen, alcohols, cardiovascular drugs (especially digoxin, calcium channel blockers and beta blockers), iron, cyanide, illicit drugs, methemoglobinemia, insulin, and carbon monoxide. There will always be more that you can know, but knowing these is an exceptional foundation I think.
Cheers,
TL