Im scared how do i aproach him?

okay im on my older sisters yahoo because i dont have 1 but i like this guy he's a senior and i have this huge crush on him. i see him alot. but i really wanna know how can i start up a conversation with him so i can ask him does he want to go to my party coming up? please help me!!!!!!!!!!!!!!! i shy when iit comes to talking to guys i really like. o and my names ariel