Don't make a huge deal out of it. Like honestly what I did is I sat them down, and I just said, "hey, I wanna pierce and do body art when I grow up". If they're cool with it, awesome! If not, just gently explain to them how much it would mean to you.
Just tell them straight-up "<whatever you have chosen> is what I want to do with my life."
But remember that's what you NOW believe you want to do with your life. As you get into it you may find it's not right for you, of you might find along the way there's something you'd like even better. So never say "That's what I'm going to do and that's all there is to it," instead say "I'm going to give it my best shot and see if it makes me as happy as I think it will."
it's your life not theirs, your supposed to make your own mistakes and your own successes, just tell them what you want to do their reaction may very, but like I said, it's YOUR life and you decide what happens in the next chapter.
tell them its your life your choces and your mistakes and they cant stop you once you start. dont let them change you you will only be with them for a little while they stop doing things for you and your on your own and thats when everything gets messed up because they told you what you wanted to do you couldnt do =)