If you are struggling to run a 100 metres, then you are unfit. One way to rectify this would be to exercise. There are lots of courses in fitness available but if you can not follow a fitness course on your own, then a wise move would be to use a fitness trainer.
Alternatively if you excel in fitness and motivating others, then it is possible to go on a personal trainer course, after completing the course, you gain a certificate and can start helping others to lead a healthier lifestyle.