I want to automate testing of flex application. I have heard that Flex provides with classes that help you automate the testing. But I don\'t know where to find them and how
To add to what Florian just said, that is one kind of testing which works with the Automation library. Personally, that is not a kind of testing I normally do unless you create a lot of custom components and you need to test their behavior.
The testing you're probably referring to is Unit Testing. In this case, there are several libraries available, but the one supported by Flash Builder is called Flex Unit 4. In this kind of testing, we're testing functions to make sure the output is correct depending on the input.
You can find more information about it on their website. If you want automation, you're going to need an automation engine like TeamCity, or Hudson to run an Ant or Maven build. If you're using Maven, I would recommend you look at FlexMojos for automated build and testing.