
Of course, the first step is to get a proper certification and select an area (or areas) of training to perform in. Getting more education and certifications can help improve their chances to get a job, but the more important thing is that give them a real perspective about their qualification and what sort of jobs are available on the market.
Consider this: at one time the athletes were the only people using personal trainers to help them excel at their sport. Today, from some decades, the personal trainers have been used by a lot of people (from celebrities to senior citizens) in order to help them learn how to exercise, lose weight (the fat and overweight people are numbered in tens of millions all over the world) and adopt a healthier lifestyle.
As you already get it, personal trainer's basic job is to teach people how to exercise. But it became something more, because most people are lazy: the trainer mut be an educator, a motivator, a coach or even, sometimes, a friend to his clients. It's hard to do all these thing working in a gym, while working with the general public, but it is an option for what can be an interesting job. As any gym, some people already know what they are doing, but they may need some extra guidance and supervision when working with specific machines or performing a new exercise. Others who have no experience, in order to obtain it faster and without accidents (that may happen without supervision) they may need to work with a personal trainer for the first few visits - the learning curve is always faster when you have near someone that knows exactly what to do.

The athleths need specialized workouts, so the personal trainers specialized to work with them are the athletic trainers. They know the difference between the diet and exercises dedicated for runners, basketball or football players, and so on, as the trainers normally choose a specific sport and focus on that area of interes.
Exercises and diet are also used for rehabilitate a person after an injury, a surgery or other health related issues (for example a stroke). Usually that kind of things that almost cripple a person, and he (or she) need a lot of time and effort to recover and regain mobility. The personal trainers specialized in these sort of exercises are called rehabilitation specialists and they have to help rebuilting the muscles and the state of mind.