Learning edible wild plants takes time and effort, there are very few shortcuts. However, the following tips will save you a lot of wasted effort. If you are serious about learning your local edible flora here what you want to do:
The Terrible Secret About Most Edible Plant Books
I get several e-mails a week asking "what one book is the best guide to edible wild plants?". The quick response is Peterson's Guide to Edible Wild Plants. It is probably the most comprehensive guide to edible plants in North America even though it mainly focuses on the northeast. It has color pictures, line drawings, and habitat info on hundreds of plants. But I have yet to meet anyone who successfully taught themselves more than 6-9 plants using this book. Few people can translate its line drawings and postage stamp-size pictures into real plants in the real world.
There's no such thing as a prefect, complete edible plant guide. Unless you are already a plant expert it's impossible to teach yourself all your local edible plant from a single book. It's too hard to have clear pictures of every plant in every stage of it's life. For that reason you really need to have multiple reference books. I have over thirty plant books that I use as guides. These aren't all just about wild edibles. They also include wildflower guides, weed guides, tree guides, botany textbooks, gardening books, forestry books, etc... Each book has different pictures and descriptions of the same plants. Once you get some books start flipping through them every chance you get. You want to train you eyes to see specific plants in all their stages among the Big Green Sea that surrounds you. You don't need to know the name or anything else about the plant at this point, just that you might have seen it in one of your books.
Update: when this article was first written I never expected I'd write a book. Well, I have and it avoids all the problems I described. Idiot's Guide: Foraging have multiple, big pictures of each plant (leaf, flower, stem, fruit, etc...) along with pictures of any poisonous mimics. It includes range maps to show you where in North America each plant is found, it even has a calendar showing you when the plant will appear depending on if you're in the south, central, or northern areas of this continent. Idiot's Guide: Foraging is available in both paperback and Kindle editions.
Once you've found a plant that might be edible it's time to ID it. Take a bunch of pictures of the plant's flowers, leaves (top and underside), stem, and overall appearance. Compare it to many pictures in your books, match the leaves, it's size and shape, and where it is usually found to similar plants in your books. At this point it's very helpful to understand plant descriptors (sepals, palmate, lobed, etc...) as it makes it easier to search through the books. Don't limit yourself to just using books to ID a plant. The internet is obviously another great resource for figuring out what the plant might be.
Take a Class
The best thing a plant newbie can do is take a class and I'm not just saying that because I teach the subject. A few hours with a good teacher will get you through the first, steepest part of the learning curves. By the end of the class you won't be adrift in the Big Green. You'll be able to pick out many plant all around you that are safe to eat (as well as know which tasty-looking plants are highly toxic!). Once you've been taught a bunch of edible plants, learning more becomes much easier as your "plant eye" will be much stronger. Then when you are on your own looking at a landscape you'll already see plants that you can/can not eat. You'll be left with just a few plants that you don't know, which is no longer overwhelming.
The other nice thing about taking a class is you'll get to see plants in different stages of their life. A particular plant may not be ready for harvest yet, but by seeing a young one you'll be able to go back and follow it's growth. Or if it's past time you may be able to collect seeds to grow your own.
Growing Your Own
One of the best things you can do to learn edible wild plants is to grow your own. Seeds can be either collected in the wild (follow all appropriate laws!) or purchased via the internet. Observing the plant from seedling to maturity is a great way to train your eyes to see it out in the wild.
The Well-Trained Eye
The repeated scanning of your plant books, internet sites, and home-grown plants will have filled your subconscious with key plant-shapes to look for and you'll be surprised at how they suddenly jump out at you! Each time you go out pick a few new plants to research and after a year or three you'll have mastered the all local edibles. You know you are doing it right when you start dreaming about edible wild plants.