Through some organisation like YFU or AFS?
You'll also be staying in a host family?
School's like this (at least it was for me when I was in Japan):
Homeroom starts around 8:35
Classes start at 8:50 and end at 9:40 (50 minutes a class)
In between classes you have a 10 minute break (9:40 - 9:50 etc.)
Lunch is at 12:40 till 13:20
Your host mother(?) usually creates your obentou (lunchbox) everyday for you. You eat that in lunchtime.
In the beginning you'll probably eat alone if you don't have the courage to talk to other students and sit with them to eat lunch, as they all form groups to sit with eachother eating lunch. This can be either in classroom or the cafetaria if the school has one. (The school I was on didn't had a roof you could go to as you often see in animes, which was too bad.)
After that classes are from 13:25 till 14:15 and 14:25 till 15:15
Classes on my school:
Classic (Japanese Literature)
English reading
World History
Kendo (it was also a club activity you could do, but in the first year it was a class I had to follow)
English grammar
English communication
Information (IT class)
Math one
Science
Biology
Math A (different than math one)
PE
Music/art/calligraphy (you choose one at the beginning of the year and you go to that class every week for 2 hours one day - only in the first year it was for me)
Society
Japanese Language
Long homeroom (once a week)
Last homeroom is from 15:20 till 15:25
Club activities start at 16:00 till 19:00 (yes, 3 hours every day, usually on saturday there are games against other schools if you join a sports club)
I've done both basketball and soccer. I figured I sucked at both but I liked sporting so I kept playing soccer even though everyone knew I sucked
In the beginning it might be difficult at school because you might not speak the language at all, even less understand what's going on.
In the classes I didn't understand (which was practically all but PE and english) I studied Japanese at the library either with a teacher or alone. I did this the whole year as I couldn't speak it when I first came to Japan and still didn't know enough at the end of the year to understand what the teacher was talking about in for example biology
Making friends was also pretty difficult as they are close-minded compared to western students at the same age. It's near impossible to know their true feelings and instead they just lie to not make you feel dissplesant or anything. This could be frustrating in the beginning, but the longer you stay in Japan the more you get used to indirectly speaking what you think.
I went to Japan through the organisation AFS. In the province I was (miyagi-ken / Sendai) there were few more foreign students from the same organisation. In the beginning you'll probably spend alot of time with them instead of with your friends at school and host family, depending on the situation in your home family and at school.
Important is to stay optimistic and smile
Anyways I could tell alot more if wanted, but I hope this gives some basic info about high school life in Japan