See the Wisdom section for more instructions, hints and tips.
Pretend that someone just gave you a car - your very first car.
You'd be proud of it ... take care of it ... change the oil ... wash and wax it.
If you don't take care of it, it will break down. Repairs are liable to be expensive and you may have to buy another car, if you can afford it, or do without a car.
You decide that it is better to take care of your car than do without a car.
The same thing applies to your body. God gave you a body and it is up to you to take care of it because it's the only one you have.
But how do you do that?
By eating the foods that are good for you (a wide varity and lots of fruits and vegetables) and getting frequent exercise. The current recommendations are two and a half hours of exercise every week. You should take that as a lower minimum.
You need both weight training and cardio like walking, running, or bike riding.
Make eating healthy a habit.
Make exercise a priority in your life. Make it a habit, too.
And don't wait to get started. The sooner you start taking care of your body the more it will pay off later. As you get older you'll be glad you did!