To do anything... anything in your life, you have to be healthy.
How to be active and fresh?
Brain needs energy, oxygen & blood supply. How to get it?
Simple, when you do exercise, heart pumps the blood and brain gets enough blood, oxygen and the energy and then you will be active and fresh.
Observe your body. When you are sick you need doctor, doing exercise will not help that much, to know that you should know your body, so observe your body.
Do we have to exercise a lot, to be active & fresh & healthy?
Nope, 15-20 minutes daily exercise is also GOOD. Whenever you feel dull or want some energy, do little exercise. If possible do 2-3 times, `15 minutes` exercise everyday.
So the tool is the exercise. Remember when you do exercise, you need to eat enough after some time, don't eat immediately after the exercise.