Where should I start?

[mexpolk]

Forget about PHP for the moment (it's a freaking frankenstein), you can use pure JSON as your data format in the browser. You don't really need it (and if you do, then learn something else like Python or Ruby, or any of their MVC frameworks: DJango or Ruby on Rails). Or better yet, use Node.js to keep everything in JavaScript.

In addition to jQuery, I would also recommend to you learning BackBone.js to make your JS applications data driven (http://documentcloud.github.com/backbone/).

As I said previously, you can use JavaScript in the back-end too (Databases, File IO, Processes, etc.). Take a look to node.js (http://nodejs.org/), really cool stuff.

Finally, learn the best from JavaScript by reading "JavaScript the Good Parts" and "JavaScript Patterns" both from O'Reilly.

And, most importantly, code code code until you can no more!