The Yahoo! Web Developer Network provides a one page overview of JSON, giving a quick tutorial on JSON, how to get Yahoo! Web services to emit JSON, offering an output of a JSON object literal as well as using a callback function method. Yahoo! also describes how their web services typically translate their XML structures into JSON.
Ryan Campbell of Particletree offers a step-by-step guide to getting started with JSON. Since JavaScript can use JSON to either send or receive from the server (both parts of an AJAX request), Ryan suggests getting started with sending JSON (after stringifying it) to the server and converting it to a server side object. Then sending that object back, encoded as JSON, to the client and process that. He offers working library code for both JavaScript and PHP (for the server part).