Brief Answer:
daily exercise, good control of diet, a healthy lifestyle is a must...

Detailed Answer:
Hello there,
Welcome to 'Ask A Doctor' service...
I've gone through your query...

To improve fitness, you must adopt a healthy lifestyle first...

This includes: not to take any unnecessary stress,
eat and sleep in time,
a good night's sleep of 6-8hrs per day,
Have mostly homemade foods,
not to indulge in any form of smoking/alcohol
avoid fast/junk/processed foods
not to spend too long in front of the TV/computers, take frequent breaks
-whenever possible

Along with these, you should start freehand exercises, jogging for 30-40min daily...

Do check yourself routinely by a physician and undergo routine health check-ups as and when required/advised...

All these will be helpful in improving fitness as well as maintaining a healthy life...

In good health...

Take Care
Kind Regards

---