How do i get over being afraid of scary stuff?

i'm 13 and whenever i see a scary movie or hear a scary story or even get a scary chain letter i'm always scared senseless for at least a few days. then i get over it but i hate being scared. one time just the poster for a movie scared me. its pathetic. like today i went to the movies and i saw half of the preview for the uninvited and when the preview for the unborn played i covered my eyes but i could still hear and i opened my eyes for a split second and i saw this creepy kid ghost and now i'm flipping out.