I've heard that your metabolism starts to diminish over time. what types of food should i eat and and exercises to maintain a strong metabolism?

Exercise is key: Exercise, not nutrition, is key to maintaining your metabolism. With that said, you can overwhelm an active metabolism by eating junk food. You already know basics: plenty of fruits & veggies, lean proteins, healthy fats & oils, etc. Avoid processed & fast foods. Check out mediterranean diet - it's got plenty of evidence supporting long-term benefit. And keep exercising! strength/aerobic/stretchin.

High Quality: Avoid processed, packaged, and restaurant food most of the time. Lots of veggies, very light on fatty foods and especially red meat. As for exercise; some exercise is slightly better for you in terms of weight loss, injury, etc. But my advice is do the exercise you enjoy, because doing it often if the key!