I started teaching fitness classes about 20 years ago back in college, and had to take both a written exam and a practical at that time. Having to go through each really helped me instruct others because 1. I understood the body and how it functions in relation to movement and 2. I was able to practice teaching before I ever had my own class.
It will take some time for you to feel confident, but that just comes with experience. I will tell you that I teach both group fitness classes and small group classes. In my group fitness classes, I lead and do the exercises with them to the beat of music. I find this format harder to teach because you have to have coordination, watch the class and give modifications, and be able to cue the class all at the same time. If you choose to teach these kinds of classes, I would suggest taking some classes from good instructors and finding a mentor. It really helps.
If you teach small group training classes, you are more of a coach. Feel confident that you know the exercises and how to demonstrate them. Take another trainer’s classes to see what works and what doesn’t. Above all, have a carefully written plan for what you will teach. That always helps me feel more confident.
Teaching classes helped me to transition to personal training, and I feel (personally), that being both a fitness instructor and a personal trainer has been a valuable combination. Personal training is obviously one-on-one, but having my group fitness background really helped me with both. It helps with scanning the room quickly, watching numerous people at one time, and looking out for those who need modifications.
Good luck to you, and try finding a good mentor or two that can help you. One other thing is you can always practice with friends and family to gain some experience. You have to start somewhere!