In order to show an interest in someone, it may start with a brief eye contact lasting a second or so. If the particular person holds the eye contact or he looks away and looks back again it is a good sign he is interested.
In anything the most difficult part is to start something. Flirting tips for shy girls will work only if you initiate it. Perhaps a guy will look at you. He might look several times. Find courage and look back. A little smile will do a miracle.