From my experience

1. I have gone through the following link first and observe the sample application(it comes with the source code).......just download and run it....(get read me file to setup the script)

http://www.adobe.com/devnet/flex/?tab:samples=1

2. choose any one server side language that u know(to club with flex). for example to club with dot net (http://www.adobe.com/devnet/flex/flex_net.html). or java whatever u want those links are available on samples page.

3. Use Adobe Flex Help(comes along with flex 3 builder on help section). just search to get any source code sample.

4. once you passed the beginning stage, use the book called FLEX 3 COOKBOOK to learn real world scenarios(code snippets) while coding.

5. Last but not least.....u need to know something that flex mother tongue language is XML :) ......any language(.net,java,php,ruby) should speak up with Flex using (XML) language.

Good Luck!!