Like everyone else said apprenticeship is the way to go. It might take a while to find someone to take you on, but eventually you will. It took me 2 years of waiting and dealing with BS to finally get into a good, well run shop and start mine. All I have to say is don't give up and keep practicing different styles and keep getting better.
I also suggest taking a look at some tattoo artist's work and looking at their line work, shading, color work, anything you can. Just study their work and practice (line work especially). Some good people to check out that are world renowned: my buddy Halo Jankowski, Mike DeVries, Jeff Gogue (my favorite), Tim Hendricks (amazing work!), Joe Capobianco, Bob Tyrrell (amazing black and grey tattoo artist), and there's many more. But these guys are just a few of the best IMO.
Any other questions feel free to ask!