The best place to start is with a REPUTABLE book on edible plants. Take it with hiking and playing outdoors and see what you can find. Everything is seasonal and regional so if you start with your area and expand, you'll be an expert quickly.
If you're interested in survival, tubers and roots are going to be a great source of carbs (starches) Wild Carrot (Queen Anne's Lace) and Indian Cucumber jump to mind quickly. Also wild alliums (Onion, leek, garlic) are very easy to distinguish.
J-
Just a caution - both Wild Carrot (Daucus carota) and Wild Onions (Allium spp.) have poisonous look-alikes that are extremely poisonous - Poison Hemlock (Conium maculatum) is a Wild Carrot look-alike and Death Camas (Zigadenus spp.) is a Wild Onion look-alike.
And don't go by the 'if it smells like an onion, it's safe'. While it's true that if it smells like an onion, it is - after you handle an onion or two, everything, in your hand, will smell like an onion, even your SAK.
And another thing, don't rely on just one book - I have one book that says "The blue berries of Woodbine are edible."(Virginia Creeper-Parthenocissus quinquefolia) [Guide To Northeastern Wild Edibles, E. Barrie Kavasch, Hancock House, 1994, ISBN# 0-88839-090-4, page 16], another (Venomous Animals & Poisonous Plants, Steven Foster / Roger Caras, Peterson Guides, 1994, ISBN# 0-395-35292-4, page 190) that says "Results ranging from gastric upset to death have been reported."
A book that says Mock Strawberry (Duchesnea indica) 'can be fatal' (The Complete Wilderness Survival Manual, Hugh McManners, Macmillan, 1994, ISBN# 0- 7715-9035-0, page 89), another that says they're edible (The Encyclopedia of Edible Plants of North America, François Couplan, Keats, 1998, ISBN# 0-87983-821-3, page 210).
The point is, to be as safe as possible, if you're learning from books, use
several, and cross-reference all.
Just my 2 cents.
Doc
and eat the whole lot in a day!