There is a really common mistake that many people make without realizing it, that might actually be slowing down your metabolism: forgetting to work out.
Eating well is only half the battle to losing weight or maintaining a healthy shape. If you forget to work out while trying to eat healthy foods, you can still risk slowing your metabolism.
Exercising helps boost the metabolism because muscle cells need a lot of energy. Strengthening your muscles can cause your body to metabolize food into energy faster to help sustain those muscles.
Cardio is the best type of exercise for boosting metabolism. It also helps burn calories and can even reduce appetite. Try anything that requires your heart rate to go up - running, biking, dancing, even walking for long periods of time counts as cardio.
Strength training can also help boost metabolism, since it tones your muscles to increase lean tissue mass. There are a ton of workout videos online to guide you in strengthening your muscles.