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).
Ryan Campbell shows how to get the Prototype library up and running, and introduces and documents Protoype's $() function. Form helper functions, getElementsByClassName functions, Element helper functions, Ajax support, timer functions are all covered.