How can i talk to a girl that is older(she's 18)?

i really like this girl but I'm 16 and she is 18 this is her last year in my school, and i really want to talk to her but my only problem is that i don't know how speak to her for 1st time cause i get too nervous ( this only happens with this girl ), i have never talked to her but she do know i exist, With what should i start speaking? i tried to keep looking her when we have eye contact but i can't i get too nervous. I really want to get to know her. What should be my first word i should say after ';Hi';?How can i talk to a girl that is older(she's 18)?
The best thing to do is just try telling her that you think she is real pretty and seeing how it goes from there. Just don't try to jump into a relationship. Try to be friends first and then trying to get with her when you two are alone.