Qualities of an Instructor
The first, and maybe the most obvious, question to ask yourself is; are you good instructor material? It is worth taking a pause to consider this. Not everybody is cut out to be a karate instructor and those that are not up to it would be better off staying as a student rather than inflicting bad teaching on a new generation.
Experience and Ability
An instructor should have experience in the martial art that they are proposing to teach and be knowledgeable about the techniques that the art is comprised of. Students will be coming to you as an expert and a teacher. You would not expect to pay for guitar lessons from somebody who had never played the instrument and the same applies with martial arts.
Not everybody believes in the belt system but it is usually a good indicator of the proficiency of the martial artist so in my opinion an instructor should at least hold a black belt that has been awarded by a reputable association. This should ensure that the instructor has undergone a minimal amount of instruction and has demonstrated that they can perform the required techniques.
Apprenticeship and Certification
A lot of martial arts associations are protective of their reputation. They don’t want bad instructors representing them and so they have their own conditions for becoming an instructor. If your new club is going to be affiliated with an association then you need to find out what the requirements are. Some associations will expect you to have completed a minimum number of hours teaching within another class. This is a kind of apprenticeship where you have the opportunity to lead parts of the class under the supervision of another instructor. Even if this is not a requirement of the association I would recommend you ask your instructor if you can do this anyway. If nothing else it will give you a flavour of what teaching will be like and help you decide if it is right for you. Some associations may even have a formal instructors certification program with tests (both physical and written) required before you can call yourself an instructor. If this is required then take the test and get the certificate as this will give further proof to any of your prospective students that you know what you are doing.
If at all possible you should continue to be a student under your instructor while running your own club. This will ensure that you continue to learn and grow within the martial art and have new things to show your students. It also means that you can progress your own rank and maintain your fitness levels and flexibility. Teaching can be physically demanding but nowhere near as demanding as training as a student. I would suggest that if you teach once a week you should try and attend another class twice a week. If you teach twice or more a week I would still suggest that you train once a week with your instructor if possible.