I studied abroad in college and it was easily the best thing I've done in my life (so far). That's great you are already thinking about going. I had some friends that went abroad in high school and the experiences were mixed. Yes, they got to drink, explore, experience culture....all that fun stuff but it still had a "high school vibe" (if that makes sense). Some of them went again (elsewhere) in college and said the experience was much better. Was it because they were older, "experienced", more mature? Tough to say.
I'd say only go for 1-3 months in high school. Summer would be ideal. The experience would be great to put on your admissions essay as well and would likely set you apart from the majority of applicants. If you can stay on track to graduate and take a semester abroad in high school then definitely go for it.
As others have said:
Organize all the things you need EARLY (passport, VISA, living situation, phone, etc.)
Bring much less than you think you need (like one checked bag max).
Sounds cliche, but be open minded. You're suppose to feel uncomfortable sometimes.
Things will not go as planned; deal with it.
Go single. I'm just going to say it again, "do not be in a relationship when going abroad".
Go somewhere where it would be extremely difficult to visit again with a bunch of people your own age.
Spanish speaking countries to study in (suggested by friends who have been):
Madrid, Spain (really anywhere in Spain)
PM me if you want to discuss more.