My diet is causing me problems. why?

For years now this has been happening and have only been fully aware of it the last few months as I developed an interest in fitness. For breakfast I would have a bowl of some cereal..then later I would have 2 slices of toast with butter. then later again I would get soup. then a piece of fruit or 2. then later probably whatever I could find like oven chips or pizzas or whatever u get the picture. but when I exercise I would feel so sick and weak that I wouldn't be able to continue or I would get sick. but if I ever got a decent meal into me this would never happen.and I rarely get a decent meal. what foods should I buy to get healthy?