How do you tell a guy you've liked him for a long time?

We go to the same high school and we've just graduated and we have our senior prom next week. We have never spoken since we have never had the same classes or group of friends. I think i will tell him how i feel on prom probably at the after party. He has a date though i know they are just going as friends. Im thinkingnwhat the heck, i will never see him ever again why not just tell him how i feel. But i cant seem to find a good sentence to say without looking like a creep or scaring him off. I was thinking " hi i'm sarah, you probaboy dont know me but im liked you for quite a while now". Though theres no way of answering the question without feeling akward. Any suggestions? Ps i want a serious relationship and not a hookup and ive heard he wants the same thing. Thanks for your help :)