The best foods for the task are fish, green leaf veggies, whole grains, and fresh fruit. However, to speed your metabolism up, an all around balanced diet with very few fats and sugars is necessary. Eat healthy, stay hydrated, and exercise on a regular basis. Exercising helps build lean muscle, which speeds your metabolism.
Generally, food can increase a slow metabolism by increasing the frequency. The food choice is not as important, although it is important to choose foods with high protein content. Try to eat every 3 hours for maximal metabolism. This helps the body not store fat.