If you're interested in helping others learn on Khan Academy, you'll want to create a free account for yourself. Once you have an account, you can "coach" other Khan Academy users. Being someone's coach gives you access to their learning data and allows you to recommend material for them to study.
To create your account, go to khanacademy.org. Every Khan Academy account is both a learner account and a "coach" account, so you can use any of the sign-up options. However, if you click "Teachers, start here" or "Parents, start here," the site will also guide you through the process of adding students or children.
Once you've created your account, you can access Khan Academy's coaching tools by clicking COACH option at the top left of your screen.