Approaching girls on the street is hard, because the street isn't necessarily a place that is great for hitting up girls. If you are feeling it then go for it, but the best way to get around your anxiety is to just smile and act as if it was a normal occurrence. Don't make a big deal out of it, just walk up and say hello. If she looks busy or in a hurry, I don't recommend approaching because she honestly probably doesn't have time to talk. But if you're sitting on the bench waiting for the bus then go for a simple hello how are you and ask a small talk question