Tattooing is a vocation that attracts many but is mastered by few. Often there is a lot of glam and drama that television shows and such bring to a career that is often grueling, unfulfilled, and financially insecure. That being said those that can stick out the worst of it often enjoy their jobs immensely. This is something that is a passion and requires a lifelong commitment as well as learning. Simply deciding to be an artist and buying a machine doesn't make you a tattoo artist, it makes you an idiot with a machine. Though there are some self taught geniuses out there most have worked very hard to learn their craft and have spent years working with more experienced artists to get there.
The first lesson in how to learn to be a tattoo artist is to be humble. The day you think you know everything you should walk away because you will never get any better with that attitude. You are the instrument that is permanently changing someones life, and you should be humble, because out of however many thousands of others they could have chosen they chose you. Understanding that is a pretty life affirming moment in learning to tattoo. It isn't just about slapping an image in skin and getting paid, especially not to the person getting tattooed; even if getting paid is the name of the game in the end. If you want to learn how to be a tattoo artist you have to be humble because you're going to need to beg and work from the bottom to get a foot in the industry if you're lucky.
The second lesson in how to learn to be a tattoo artist is to have some persistence. The first lesson I always tell apprentices is show up. Any monkey can be taught to draw a picture or copy from a page, but the hardest thing is showing up for a job you don't get paid for and doing the grunt work. You will get refused apprenticeships. If you plan on doing this the right way then you know it's a long road and it starts with finding someone you admire and begging them to teach you. This is an art career, and if the thought of drawing 12 hours a day 6 days a week doesn't bother you then you might have a chance. Draw, draw, draw, and draw some more before you ever even ask for an apprenticeship; but make sure they are quality examples of how good you think you are and not a hundred 5 second hashed in sketches you did in the car before you walk in the door.
Third and probably most important is don't expect to tattoo immediately. It's tempting to try and skip to using a tattoo school, or a paid apprenticeship simply because you think it will get you there sooner. Learning takes time, usually it can be 6 months before you touch a human or even hold a tattoo machine if you aren't devoting a lot of time to this. Grunt work is normal, remember the karate kid. Learning to tattoo in a week and showing up with a “certificate” will get you laughed out the door of any self respecting studio, so don't waste your money. The same goes for buying machines and equipment, until you know how to tattoo you don't know what you prefer to use so don't waste your money.
Finally if you still want to learn to be a tattoo artist remember that your life will no longer be your own, the hours are long and often include evenings and weekends. Find someone you admire and prove you are worthy of their teaching,