From across the room, smile and keep eye contact. If you don't know the shy person you're interested in, start making modest attempts to get his attention. Try smiling at him during class or during lunch, and make 2 or 3 seconds of eye contact before looking away. This will show that you are not only interested, but also not trying to be creepy. If you are lucky enough to know the shy person, then help him out by giving him your attention. Make sure you are both sitting near each other! Invite him to sit with you or join you after class/lunch, and talk with him. Don't just sit there in silence - say something funny or interesting. Shy guys like to be asked out, so go for it! And if he says no, that's fine too - you didn't lose anything by asking.