Taking a personal trainer course is the first step you need to do if you wish to start working in this field. There are many different associations that offer a personal trainer course so you should be able to find one that is targeted towards what you are hoping to achieve with your career.
Often there is a personal trainer course for resistance training, group exercise, aerobic exercise and aquafit classes. By taking a variety of them you will greatly increase the areas in which you will be able to work.
The nice thing about this too is that then you will not have to deal with the same type of activities every day and can focus on numerous areas of health and fitness. This will help to prevent burn-out as an instructor and will also increase your knowledge base tremendously.
You will likely learn about many different aspects of fitness during your learning sessions, including things like anatomy, physiology, exercise biomechanics and nutrition. You may also even do a small amount of work learning about the psychology behind health so that when it comes time you can motivate the clients you work with and help them solve any potential psychological issues that stand between them and their goals.
It's always a good idea to check with the company you wish to work for to see which organization they prefer you to have your certification through. This will help to ensure that you are able to find employment where you desire after finishing. Some may be very specific with what they are looking for and others will not be.
By looking into it though you prevent any potential problems and ensure the time you spend learning is well worth your effort.