Anybody can attempt to teach anybody anything they want.
What credentials you have will get you a foot in the door.
That being said, credentials up the ying yang do not an instrucor make. At least one you can learn something from.
I have seen some instructors with no formal training that were better than any instruction I had in the Navy while I have seen some instructors with years of training that were so boring you couldn't learn a thing from them.
Credentials are important to some factions when it comes to you giving credentials for having the student take your course of training.
For a course that you think you can really help someone learn and you can convince them it will be beneficial to them, your credentials are your confidence.
If you plan on making money doing this you will need a business license in most venues although some don't require one. You will still need to file income taxes for what you bring in, unless you're doing it for free.
If you are talking about Concealed Weapons Training for a permit, you most certainly need credentials before you can give a student a course completion certificate.
KK