If I had to claim a "gateway" plant for my foraging habit, it would have to be the wood sorrel, which we used to jokingly call "sourgrass". Wood sorrel has a pleasantly tangy taste, and I would typically just eat it out of hand while weeding the garden as a kid. The plant (genus Oxalis) contains good quantities of vitamin C but also contains oxalic acid, also found in spinach, which is toxic in large quantities (it just goes to show you that you have to eat a wide variety of foodstuffs). As luck would have it, it's growing in profusion on one of the sites I work.
Once again, the Wildman is the go-to guy on foraged vegetable matter. Green Deane's "Eat the Weeds" series is also an invaluable resource:
Of course, the wood sorrel looks a lot like clover, which is also edible, though I have to confess, I've only eaten clover blossoms. In Jamaica, "sorrel" refers to the red hibiscus, which is used to make a delicious punch (I remember having to bite my tongue to keep from laughing when I received a bottle of "Ho-Made" brand sorrel punch at a local Jamaican restaurant a number of years ago. I kept the bottle for a while, but have misplaced it in a succession of moves over the years.