Do I need a degree/diploma/certificate to start as a fitness instructor?

Do I need a degree/diploma/certificate to start as a fitness instructor?

Though it is not, it certainly gives you an edge over others. And it is not only about giving edge over others, undergoing a professional training also helps you in socializing with people who have same interest as you. Your mentor and institute help you big time in bringing your dream to reality.
Besides, almost all reputed gyms, fitness clubs and chains employ only those trainers who have undergone a professional training. Training not only acquaints you with the practical aspects of the job, which many people mistake to be the only knowledge which is necessary and required, but also teaches you theoretical aspects of it, which are equally important, such as human anatomy, a general knowledge about various ailments, etc.

Additionally there are certain domains such as yogic trainings, naturopathy, aerobics, etc. which essentially require training before going on the floor with the learners.