I did an assignment about this once, so I've put a lot of thought into it. Interesting how it puts your life into prespective, and makes you realize your priorities.
Anyway,
I would tie up some loose ends that I'm faced with right now, and tell my mom, becauce she'd be detached enough to take it well and not overreact, but caring enough to give me advice. I know she'd keep the secret well enough to let it last a long time.
I'm at a great place in my life where I don't have any payments to make, so this year I would devote my time to do volunteer work and save some money.
I don't think I'd still go to college, since I've always had sort of a vision that I'd eventually detach my life from our society's structure.
After spending time with my family at Christmas, I would announce that I would be spening the next year travelling. I wouldn't tell them my situation, because there would be too many contrasting reactions and opinions to deal with. I don't know where I'd go for sure, but just take it one destination at a time.
My life for that year (or so, depending on what happens) would be devoted to seeing the world and having an adventure. I'd help anyone along the way that needed it, and take tons of pictures. I guess kind of like a modern day Johnny Appleseed. I'd spend the next 3 years back at home reflecting, writing, and creating, saving as much of myself for the future as possible in case anyone was interested. I would probably try to learn an instrument. I'd let the rest of my immediate family in on the secret when appropriate. Not too far into my last year, I'd let my family know. It would spread like wildfire, but it would be OK, since by then I'd have hopefully prepared myself.
I'd spend the last year of my life with my family as much as possible. I'd also prepare a letter and a gift for each of my family members and close friends, letting them konw how much they meant to me, let them know what memories we shared and what secrets I always wanted to share with them (but I'd still take most of them to the grave, I'm sure.) After that, I'd give away all that I had, and then I'd be ready to pass on.