How can I flatten my stomach at 41 years old?

I am 41 years old and haven't exercised for over ten years, although I do walk everyday for about 20 mins to and from work. I am not overweight (60 kgs), and eat healthy every day, however, I do also indulge in chocolate and potato chips and fast food on a weekly basis... I am showing signs of cellulite for the first time on my arms and stomach area (under certain light) and my upper thighs... My stomach has never been flat, although when I stand it looks fine. How can I make it flat at home, through exercise? When I sit, you can grab a whole chunk of skin. I hate it... Can anyone provide proper advice about something that will really work? How can I get a flat stomach if I am not overweight? It really is big when I sit and I am not being paranoid. I can't stand the rolls and just want to feel comfortable naked again.