[tommasi]

Be fluent in Japanese, have a permanent visa, finish Japanese university and do internship at local schools for 2 weeks, and then pass the govt exam to become a teacher.

You might have a better chance of becoming a teacher at international schools in Japan but even that will require you to have a 4 year degree, internship so you could take credentials at university, and have few years of experience teaching.

That should be a starting point for any international schools in Japan but expect high competition.