First of all, you should start taking yoga classes which will teach you the basics of yoga. You should find a suitable yoga centre or academy near to your location that offers yoga classes.
You should purchase different books of yoga which are written by yoga experts who have an extensive experience in this field.
Reading different books will help you to gain knowledge along with practicing the moves of yoga. It will also help you in understanding the class lessons more effectively.
Make sure you have mastered the major types of yoga and know all poses which are used in these types. It will help you to become a good yoga instructor.
Now, after getting knowledge of different types of yoga like Ashtanga Yoga, Bikram Yoga and Iyengar Yoga etc, you should choose the one that is most interesting for you. Deciding the type of your interest will help you in becoming a good yoga instructor.
After selecting a specific type, you should get training to master the poses of that type of yoga. Try your best to consume as much time as possible in practicing the poses.
Then you should sign up for participating in yoga workshops which will help you in focusing on your techniques and will make you a good yoga instructor.
After getting the required training, you should know if there is any certification involved to start your own yoga classes. You have to follow local regulations in this regard as without it people will not come to you.