if you're trying to build on your frame, it's good start raising your intake of complex carbohydrates and a proportionate amount of protein. Try to have a serving of lean meat the size of a deck of cards on a daily basis with 125-250g of brown rice or a baked potato for a typical meal. Good snacks usually include nuts and fruit.
It's also not so much about what you eat but WHEN you eat. Your body will respond best after recent exercise, so make sure you eat well after you work out.
