Almost all the information you need whilst working with yii is written in the Class Reference.
The point is just to know how to find what you need.
To find this information, you have to understand what is Yii::app() and what is "this" in the context in wich you are.
In each point of your application code (view, controller, widget, model, module) you can call Yii:app().
This will give you the actual instance of CApplication that is running.
This object is the one that is configured through config/main.php, is the backbone of Yii. So in main.php you can configure each property you see, or add new compomentes that can be invoked via Yii::app()->myComponent.
You can do whatever is written here: CApplication.
Taking a look at this page, you will notice lot if interesting stuff, like db, urlManager, numberFormatter and many other.
So, if you need to format a number, you can use the actual number formatter, calling Yii::app()->numberFormatter->
And then what? Simply click on the link named numberFormatter on the doc page, and you will be redirected to the CNumberFormatter page, where you can see all possible properties, methods and event available.
Another important element is the variable $this. It is the actual object we are implementing. So in a controller, it will be the actual controller, you can check all possible methods on this page.
Remember that when you are in a view of a controller (so not in protected/components/views but in protected/views/controllerID), $this is an instance if CController too, the one that called render or renderPartial.