Answer:
The two most important things in staying healthy are diet and exercise. Mental health is also very important, but I'm guessing this question is about physical health.
Diet: Eat more fruits and veggies, cut down on carbs and processed sugars. Drink more milk, juice, and water instead of coffee, soda, or alcohol.
Exercise: Go for a run or walk, and/or play a sport. Be active for 30 minutes a day. This helps lower your resting blood pressure and clears your mind.
Hope this helps!