[awaage]

If you know some programming, you're kinda where I was when I started learning Javascript. I just jumped in using jQuery, because I wanted to write cross-browser code. Start by looking through the jQuery plugins website, find cool plugins you want to use, and incorporate jQuery plugins into your apps. Look through the plugin code to learn how it works. Then make your own plugins when you need them. For backbone.js, the online tutorials are good. You'll get the hang of it - good luck!

[aberatiu]

One silly question: Can a full app be created using only backbone.js, HTML and CSS? Something like a multiplayer game, for example?

[awaage]

Yes - it really depends on your needs. For example, i have recently done a ruby on rails backend, with backbone.js for frontend app. You can use PHP for backend, with backbone for frontend. In fact, that's a great place for you to start learning. Backbone.js will be used for your client-side code that runs in the browser.