Selling Apps as a Teacher on a humanities/engineering visa

I have recently developed a few educational android apps in my free time, that I use at work as part of my teaching. These are just APK files at the moment stored on my personal device.

I am thinking of selling these apps on the google play store and want to make sure I do everything above board regarding Japanese immigration and taxes.

I have never engaged in any kind of side work before and have no idea what things I should do or consider.

by Joel_Cason