General High School Resources
Teaching at high school can and will be very different based on the type of school your at. The goals of high school English teaching varies greatly depending on the level of the school.
Low-level schools will be more liable to focus on the ALTs culture, games, communicative activities, and other activities that will probably be thought up entirely by the ALT and may very well have nothing to do with teaching English.
High-level schools, on the other hand, will base almost all of their English instruction on preparing students for university entrance exams. While this may involve teaching students to read higher-level texts like scientific journals, there is little to no communicative competence required for these tests, and the students' English education and ability will reflect this. As an ALT, your in-class role is liable to minor, however your role as a native English checker (for other teachers) will be rather important.
What to Teach
Expansion of this section is necessary. If you have any ideas, feel free to hit the "edit" link and dive in :)