Do a search for "Glycemic Index" ... that'll give you a good idea of what to eat and why. The lower the glycemic index, the better.

It is a very good idea to limit the amount of simple sugars in your diet. Simple sugars are things like table sugar (the stuff you put into your coffee, or on your cereal), chocolate bars, candies, sugared beverages, etc. Fruit and veggies are all right because they've got the fiber aspect to them. It's the extremely simple, heavily processed, stuff that is not good.

The reason simple sugars should be limited is because they tend to spike your blood sugar level and then, as soon as they are burnt off, which happens fairly rapidly, your blood sugar level drops ... often lower than it was before. This drop in blood sugar level triggers the hunger sensors in the brain, and next thing you know, you're eating again. It's a great way to gain weight. Also, there is some evidence that this constant spiking and dropping can cause defects in the pancreas which may lead to diabetes.

However, if you eat complex carbs, fats, and/or proteins, your blood sugar level will go up gradually, and will remain steady for a while before slowly dropping. You won't get hungery as quickly and it is easier on your body.