How to speak 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. Thank you for helping! PLEASE ANSWER.How to speak to a girl that is older(she's 18)?
I'm 19, and I'm very flirty with a 16 year old friend of mine. Eye contact is a subtle but powerful way to start, then a smile, and maybe join a club or an activity that she is into? Or perhaps find a common ground between you 2 to talk about.