How do I tell my mother I smoke?

My mother idolizes my brother and I for being such good kids. We both have good grades and a great choice of friends. My brother is 'straight-edge' and i am tired of following in his steps. My grades and friends are all the same, but i started smoking cigarettes over the summer of 09 and now i'm hooked, i don't like lying or keeping secrets from my mom, she's my best friend. How do I tell her? I'm scared she will be upset or disappointed in me.