You have to give some indication that you are interested without making yourself seem like a stalker. If you are to approach a shy girl, act kind of like a shy guy yourself. I don't mean that exactly, but talk quietly, don't be all loud and crack stupid jokes. Just light talking and clues that you like her is a great start. Go slowly. . . don't just come up and say, do you want to go out with me? Get to know her, first kind of like a friend (not exactly but you know what I mean) then as time goes on she will get more personal and it will not be as difficult for her to open up to you. But you have to make a move first. Coming from a shy girl, I never make the move.