How do I become an ALT at a Japanese high school as a German?

Hey, I'm new to this sub and also finishing my master's degree in economics soon. My plan is to go to Japan and be an ALT at a Japanese high school for like 6 or 12 months because I want to experience Japan. I would love to teach at a Japanese high school.

But how do I start? I don't know where to look, which websites to use to apply and I don't know any Japanese at all. My English is good but maybe not native level and also I would much prefer to teach at a high school. I heard that a lot of jobs are at kindergardens or elemantary schools but I don't want to work with really small kids. Also, as a German there is no JET programme currently. I've been to Japan twice before for vacation. Would be really cool if you guys could show me an avenue where to start!