Eat meat, vegetables, some fruit nuts and seeds. Eat good quality fats from sources like olive oil, coconut oil, avocados, and nuts.
Steer clear of grains of any kind, including whole grains--they turn into sugar just like any other form of grain/flour/processed food and increase your blood sugar levels, therefore raise your insulin levels and when you have an abundance (which is VERY easy to have when you are eating anything processed or low fat), get stored as fat.
It really is that simple. You don't need to exercise like a fiend, or at all for that matter. Your body composition is 80% diet related. You don't need to restrict calories or portion sizes because if you are eating no processed foods it will be difficult to eat too much. If you are eating enough fats you won't be hungry either and will find that you can go hours without eating with no il-effects because you aren't having the big spike in your blood sugar/insulin levels. Food becomes fuel and not an obsession.
Don't take my word for it, there are countless websites and books that will back up my claims with their science. Dr. Eades, Robb Wolf, Mark Sisson, the people at Whole 9, The Heart Scan blog--do your own research and then decide.