1. Follow the examples
ExtJS come with a bunch of great sample use of using widget, this provide the basic use of the component, creating plugin, etc. This is the best way to know what you can do with the component and how to implement the component by viewing the source code, if you somehow don’t understand what the code means, simply you can consult the API documentation first. Just download it first and run on your local server or you can view it online.
2. Read the ExtJS tutorial
So, you are ready to get your hand wet ? but don’t know what to do to digging depper to understanding ExtJS, I suggest you start with tutorial from the ExtJS official site. It’s cover about the getting started guide, dom query helper, combobox, form, grids, drag & drop, menus, tree, etc, even the basic of the application design and some of them provide the downloadable example source code. Although the tutorial is useing version 1.x and 2.x, yes it is compatible if you are using version 3.
3. Don’t fear to read the API
The ExtJS API documentation come with the bundle when you’re downloading ExtJS. So how this gonna be useful ? well for me it provide very useful information about the Ext core and default config options, properties, events, method for each component, it’s a good reference when you are trying to manipulating the component. Some component given the basic example of usage. Just download the ExtJS bundle and you can find it in examples folder or you can view it online
4. Consult the spesific question to the forum
When I get some trouble I always use Google first, type your spesific question on Google and most of the solution found in the ExtJS official forum and i always found the solution is very handy and easy to understand. Even some of the expert member provide advance source about ExtJS like quick search plugin, grid paging row numberer and custom themes and you can download it for free.
5. Use the good IDE
more ExtJS Resource and tutorial
Video tutorial from EXTJS.TV bunch of ExtJS Tutorial
http://defafe.com a blog about ExtJS tutorial
TuturTinular a blog who have some ExtJS tutorial
ExtJS and WordPress wordpress themes usign extjs and it’s free to download