I [15] can't stop thinking about a guy [18]

I didn't know where else to post this I just really want some advice... this guy graduated from high school and I still have 3 more years and I basically like never talked to him except one or two times when I asked him if he wanted to help me with something in school. He's the most attractive person I've ever laid my eyes on but that's not even why I liked him so much, he was kind of like me I guess. But anyways I saw him like a week ago because he visited our band practices and it just got me thinking about him again. My friend was at a party with him and he's shy and so is she but I told her I liked him and she was like "Just ask him out!" But he barely knows who I am and we've like never talked so I really can't. I can't stop thinking about this guy and I just wanted to know if anyone had any advice.And it's not illegal in our state to have sexual interactions but that's not really what I'm thinking aboutTl;dr don't know how to stop thinking about a guy who graduated from my school