Some states require hypno therapists to be licensed, there are places that teach it so you can get certified.
How to learn hypnosis?
I've used hypnosis, casually, since the 1960s. It is NOT something to be used lightly or for entertainment. There are too many risks involved and they're very real.
If you want to work in this field I'd urge you to find a reputable college and get at least a diploma so that you'll understand how it can be used and when it's contra-indicated.
Hypnotism covers a wide range of practices varying from the traditional method developed by Dr James Braid (following on from Anton Mesmer's ideas) to the method devised by American physician, Dr Milton Ericson and, beyond him, into the wonderland (or is that hocus pocus?) of NLP.