Of all the poisonous plants, many/most of them wont kill you, they will just make you sick or they will taste horrible... but enough will kill you that you should not risk it. Poison or Water hemlock and certain varieties of Monks Hood can kill you very quickly, even with just a taste.
Try to study the most common plants in your area: edible, poisonous, medicinal etc. You can always find some of those nearby. Once you know the uses of a handful of very common plants you can expand your knowledge from there.
I suggest learning about plant families and genus too instead of just individual species. Then you can make much more intelligent guesses based on the shared traits of that plants genus or family even if you dont know the exact species of a plant you've found.