How to talk to a guy who looks serious all the time?

Me and this guy at my school have been eyeing each other for two weeks now. I want to smile at him next time we make eye contact but I'm soooo scared! I'm usually an outgoing person but when it comes to the boys I like, i get crippling social anxiety. Anyways, it doesn't help that he give me this really serious look whenever i see him, i get intimidated i can't even smile I get lost in his eyes, his body. I dont have him for any classes so it's hard. He's practically a stranger and I dont know how to show him I'm interested. We're Juniors in hs btw. Help!!!