Girl keeps staring?

Actually, having completed my preliminary education in all-boys' schools, I am a total nerd when it comes to issues regarding girls. I am not saying that I do not feel anything for them, I just cant express it. Now the problem is, for last couple of times when I was riding a bus to my Uni, I noticed a pretty girl staring at me, through out the whole journey. She keeps staring even when I look toward her. What should I do? How should I tell her not make me nervous ? what do girls suggest?