16 and going to Japan alone? wow.. super ambitious. Impressive!
I went when I was 20.. other than Mexico, it was the first time I ever went international as well. Still love the place...
Anyways. Asile seats allow more freedom. Do you know what kinda aircraft it is? Is it a 777? If that's the case, a window probably won't be the end of the world, can lean up and sleep on it. 747 though, I never liked the idea of being pinned down by 2 others.
I take sleeping pills to sleep on planes... I mean, YMMV but you may not want to if it's your first time flying.
The jetlag to/from Japan will be really strange at first. Flights from the US take off noon-ish, land anytime between 2-5pm local Japanese time. If you're a morning person, then try to stay up till 8-9pm, if you're a night person (and Tokyo really comes alive at night), then fight it till 11pm or so.
Dunno what else I can say really. Where in Japan are you planning on going?