It's not easy, but here's how to find things on w3school:
Main page
http://www.w3schools.com/ on right is "Web References" Find and click JavaScript. Bottom of page under "HTML DOM Objects Reference" is "Input Checkbox object" click. Your on the Checkbox Object page and under "Checkbox Object Properties" is "checked" On that page is an example on setting the box, getting the state of the box and a link to a "Try it yourself" page.