This is for my niece-of-someone-else, almost-former housemate/protracted guest, will-eventually-just-be-a-regular-friend, almost twenty year old person. Everyone should have one.
I know you want to travel. I have a suggestion. It gives you a year to prepare yourself, which you'll want. When you're 21, start taking long weekends where you buy a plane ticket, rent a car or get on a train, and watch every episode of Anthony Bourdain's The Layover ahead of time. Give yourself longer than 48 hours for each visit - he mostly already knows every city, and that man can pack away the food. But eat your way through New York City, LA, Singapore - make a list from each episode, research the cool food blogs, get a map and go with a plan. Part of the plan should allow for throwing the plan out if circumstances and opportunities intervene. Either travel alone (not as scary as you think), or take your most adventurous, badly behaved friend. Take photos with a phone, but don't stand out as a tourist.
A few other things: don't eat at the hipster places, unless you're in Portland, where that's actually the native cuisine. Don't go to gluten-free special needs places - just hold the bread and gravy and noodles (except for rice ones) and eat like a local. Drink more than you normally would, but not so much that you wake up feeling like crap. Find out where the good coffee is, and the used bookstores. Keep a journal - the old fashioned paper kind, with shit spilled on the pages, in a ratty notebook, not a too-precious beautiful thing. (Actually, I think that last bit is essential to life. It should fit in your bag, which ought to be a cross body shoulder bag that's just function. Buy it with the notebook in hand, so it can try on the bag.) Make it big enough to accommodate a book, too.
And that's it. How to travel the world in little chunks, relatively cheaply (compared to other methods) and collect experiences. Try it out.
Or you could travel the way I have, by moving, which is a whole other thing. Long weekends are less traumatic I think.