I am not good at Japanese but can speak English.
Is it possible that I teach English in my open times?
It doesn't have to be proper Language School some thing like online Class that I can teach personally.
Where can I find student??
Try signing up as a teacher on Coursera or advertise on job boards famous in Japan. You can even post on Japan Craigslist, I have seen some postings like that. Or to start with, talk to others in your network and see if they know anyone who is looking for a one-to-one tutor. It is a good source of income right now when jobs are less due to pandemic.