It's all in willpower and portion control. I'd recommend if you can afford it to find a trainer who will motivate you to keep losing and keep working out. I'll admit I'm at the other end of the scale. I was born premature and weighed 2lbs 10oz at birth. I've been told I would always be smaller than the average person. Well last year I decided **** that and after I started my job doing fundraising I went down the street and got into training with the local boxing for fitness program (Fight Fit). My trainer is training me not only for self defense but to help me bulk up. I've put on about 15lbs of muscle and gotten really toned over the last 6 months. I might not be a massive muscle bound god but I feel a heck of a lot better about myself. I'm also taking Brazillian jiu jitsu and tai kwon do for the ground grappling and hard kick parts of self defense and since a lot of my BJJ training is strength training so I can lock in holds and chokes harder I've just become as a result a hell of a lot stronger than before and my cardio (even being a former track star in high school) has gone through the roof.
Basically my point is if someone like myself who's always been small and scrawny is able to have the willpower to push myself to my limits and beyond and work harder than I ever have before to bulk up someone that's larger like yourself with willpower and determination can lose weight and get to where you want.
Just remember what you want to look like and your goals when you train. You'll be sweating and the ab workouts will make your stomach hurt like hell but the astounding change you'll see will be oh so worth it.