I've done it twice. Once as an exchange student and am now here playing the ALT game.
My advice to you would be to set down some goals and figure out why you want to come and what you hope to accomplish while you're here. It'll help you with what programs to look at and which one you should choose.
Are you in university? Then look at your uni's exchange programs. If you're from the States, your financial aid package can actually be adjusted for your costs of studying abroad and you're eligible for whatever funding you normally get, plus you can typically borrow more to cover the additional costs.
Depending on the program and the agreement between your uni and the host uni, there may not be much additional cost compared to a normal semester. Mine worked out to be about equal in tuition, room & board, and living expenses.... the only real difference was the airfare.
And like some others have mentioned there's great scholarships. My semester ended up not costing me a dime, thanks to Freeman Asia, the JASSO scholarship, and a couple of other scholarships from my University.
I'm here as an ALT now... which if, after you finish school, feel that your Japanese is still not up to snuff, is a great way to learn the language. I have enough down time at work to get in lots of studying.... which wouldn't be true if I had a "real job." It's a lot of fun, but the job does get kind of boring after awhile. Your degree doesn't have to be education related... I think I know more ALTs who have degrees in other areas than those that actually have degrees in education... and it's not so much being great with people as it is just being a goofball in front of the kids. The more of an ass you make of yourself, the more they love you
My parents were all for me going to study abroad.... when it came time for me to come back to work... my dad was all for it... my mom flipped out (my brother had also recently moved, so she had a pretty big dose of empty nest). But I stuck to my guns... and she got over it.
I'd personally recommend coming as a student first. I had more opportunity/money to travel and go out... where as now things like getting a drivers license and maintaining my car have sucked up most of my time/money this year. If your parents are reluctant about letting you go, they might also feel better about going as a student, as your environment is typically a little bit more structured (ie living in a dorm or with a host family as opposed to by yourself), and the university is there to do some hand holding if needed.
I'd be happy to answer any questions about either one. Either way, living abroad is an amazing experience, and even if you're only 18, it doesn't hurt to sit down and look at your options.