How can i radically change my diet?

ok guy and gals i really need some advice. when i got my new job ( 14 months ago) i had been working out and was making ok progress losing weight. well long story short here i am now and im fatter than ever lol. i wont lie my diet has always been crap food. im really pushing to get back into working out like i had been but i know its useless unless i can correct my diet. all i do at work is sit in a cube and get bored which usually ends up munching on something. does anyone have any advice on how to break that habit and really radically change my diet?