i have read that you can plant peas let them grow for awhile then till them under and get nitrogen and organic matter which my garden needs badly. i have found a bunch of what looks like wild peas the flowers look just like them and they grow on kind of a vine like peas but the leaves are longer. could these do the same as legumes?

Cover crops are plants grown as a living mulch, esp under fruit trees in home garden orchards. If they are legumes as well they have the added benefit of fixing nitrogen into the soil, as John pointed out.
Green manure crops are the ones you grow to the point of being lush, then till back into the soil, preferably just before flowering. I have used old birdseed for this, oats are great, any out of date seeds etc as even though not necessarily a legume, the young lush plants will still add organic matter to the soil when tilled back in. So even if the wild peas are a weed, as long as you don't let them flower and seed, they will help. If they happen to be a legume then that's a bonus.

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The secret is the soil.