At fifteen you have all the answers. I remember. I wanted to play the drums and I still do. Now, I have a fifteen year old and he has all the answers. With that said, let it be known that you are far from having all the answers!
I hope you make it. Drawing and tattooing are the same and are very different. Keep that in mind. Figure out why! If you know this and still want to do this go to school like you plan and see if you can get an apprenticeship. There are no tattooing schools! Study all forms of art. People are as diverse as snowflakes. You will come across all kinds of requests. Don't be the one that says "I can't do that." Personally, I would say that drawing and painting are always good. When you master that, buy an airbrush and master that. If nothing else it will help you do exactly what you have already been doing (drawing and painting) but you'll be doing it in a totally different way. Just like tattooing!
Keep this in mind, too: do not strike out on your own and start scratching!
Hope to see you in the field some day!