T!ka :) says:

There aren’t any online courses because you have to learn through hands on experience, it’s the only way.
And you don’t go to school for it either.
You apprentice at a tattoo/piercing shop and then after you apprentice for a certain number of hours you get your piercing license from the state and then you are a certified piercing artist.
And also, it depends on the state, but in some states you have to take an anatomy class. At least that’s how it is where I live. To apprentice for piercings you have to be 16 and for tattoos it’s 18.