To be able to teach online with language, do you need to have certificate in order to become a certified tutor? Where do you usually learn how to teach and the materials needed.
Depends on the site/platform and what you plan to teach.
If you're teaching students who plan to take IELTS, TOEFL or TOEIC, then they'd prefer or even require a teacher who passed that test.
If you plan to teach conversational English, some would require that you've been assessed so they can have a solid determination of your skill level. That or you have solid teaching experience.
When I was teaching Koreans though, they just did an interview and that was it. I was in college (2001) and I started teaching kids (7 - 10) then eventually moved to adults -- moms and university students.
I checked out 51Talk a couple of years back and they didn't require any certification. Just that you complete their online training program.