Get yourself a Flora - a book that let you identify plants in a scientific way, rather than match plants to a (drawn) picture. Some edible plants look very much like poisonous plants (or the other way around). Difference are so minute, they can only be cut out by a step-by-step identification.
Get yourself a good book that covers edible plants, and mark the edible ones in your Flora.
Note that even though a plant might be edible, it may not be edible by you. Be careful when eating new plants and do an edibility test to see if you show any allergic reaction.