If you would like to contribute to answering student questions, simply: 

1. Navigate to the course you feel you are confident answering questions for, as an example we can use Multivariable Calculus. 

2. On the course page for Multivariable Calculus, scroll to the very bottom of the page and you should see a link to Community Questions.

3. Click on the link to Community Questions and you should see student questions that were recently asked relating to the topic (in this case, Multivariable Calculus). 

You can follow the same process for any course to find student questions. Alternatively, you should be able to see questions under any videos or articles on Khan Academy, and you can post answers there as well. 

I am glad to see that you are looking to offer answers to questions, and look forward to your contributions to Khan Academy!

Thanks!!