Plants of the mustard family are great for leafy greens. Domesticated varieties include broccoli, cabbage, kale, brussel sprouts, etc. And of course, mustard seeds, from which mustard is made. Plants of this family are easy to recognize, once you're familiar with them - the flower always has four petals, is usually yellow or white, sometimes pink or purple. the leaves of wild varieties are often peppery in taste, and best steamed, sautee or boiled. If boiling, some authorities recommend changing the water at least once, to reduce the intensity of flavor - I've never found that necessary.
There are both native and introduced species here - feel free to pick the introduced ones as much as you like - they are essentially weeds, useful only to those who enjoy them! Limit your picking of native ones.
I don't know of any mustards (Brassicae) that are poisonous.
For pictures, descriptions, recipes, etc, if you don't have a local person who can teach you, just google "mustard plant, mustard greens, mustard flowers", and check the "images" as well.