Unlike most other industries, you do not require educational qualifications to get jobs. But as the industry shows signs of getting organised, leading gyms have started hiring certified trainers and put newcomers through their own training programmes.
But the basic prerequisite is an interest in keeping one's own body fit. Most of the trainers start by going to the gyms to tone up their body and then develop an interest in the field. While you need not be Mr Mumbai to become a trainer, you cannot advise someone how to develop a good body unless you have one yourself. Most of the clients in the upmarket gyms demand personalised service and are ready to pay for it. So, communication skills are very important.