If you'd like to get certified in a math/science subject, then you may request a certification by submitting a video of yourself working on the appropriate Khan Academy test. For the SAT subject, you will instead request an honorary certification by submitting an explanation and documentation of your scores.
You can get certified in these topics by navigating to the respective subworld and going to the "Certify" page.
You can get honorary certifications to tutor in the Reading & Writing and History subworlds by submitting a certifying AP score (4 or 5) under the "Certify" page. In addition, you can gain an honorary certification in the SAT Prep subworld by submitting a certifying SAT score under the "Certify" page. Honorary certifications allow you to tutor more topics in a subworld but do not show up on your portfolio.
Keep in mind you still need to apply to be a tutor and get approved