We are what we put in our bodies. We have to be careful what we eat. Personally, I believe a Vegetarian diet is one of the best and provides both fibers and proteins. If you have extra time to spare, check out Understanding Nutrition.
Eat a hearty breakfast, moderate lunch and a light dinner and you automatically help your body to downsize. The logic behind this is apparent. You kick start your day with hearty breakfast and start working, you give a little boost midday with your moderate lunch and when it is time to relax and wind down, you eat just enough not to be hungry. While you are asleep, the body uses very less calories (unless you are muscular). Muscular individuals have a higher metabolism at rest than skinny individuals.
In terms of choice of food, aim to eat foods with higher proteins and fiber. Try to avoid fats that are solid in room temperature and also avoid sugar and minimize salt. Sugar is easy energy for body and helps put weight. Salt helps body retain water and can add the pounds.