A good fitness plan can do much to improve your physical and mental health, physical appearance, and it's a good way to use time. If you join a gym or another physical activity you can meet new people and have fun at the same time. That's just an added benefit. The main reason it is important to have a good fitness plan is to stay healthy.
Exercise will help keep your weight lower, which will put less stress on your joints. Your joints will last longer if there is less stress on them now. Your organs work in a similar way, if you treat them poorly with a bad diet, they will wear out quicker. Exercise can keep your organs healthy too by keeping your blood flow normal. You will generally feel better if you have a good fitness plan, not to mention that when you do have to run for some reason you'll be able to without passing out after the first hundred meters.
Your mental health is directly affected as well. When you look bad and physically feel bad, it's just a matter of time till you feel bad mentally as well. When you feel good physically and you feel good about yourself in general (for being able to stick to a plan, and for looking good in a mirror) you'll find it difficult to feel bad mentally.
If you're looking to create a fitness plan, this site will help give you some of the basics.
http://www.mercola.com/nutritionplan/exercise.htm
