Question: I received a .swf (Flash) file and they asked me to modify some things.
I don't know (yet) Flash.
Is a .swf itself a source file or it is a compiled form of something other.

What is the suggested IDE to work with Flash, where to starts?

Answer: 
The **swf** fileformat has not source code in Action Script (the language which you can develop in Flash).To be able to modify something in flash project you need to has access to **.fla** or **.as** files.

**.fla** — files contain source material(images, movies, fonts, code, etc...) for the Flash application. Flash authoring software can edit FLA files and compile them into .swf files. The Flash source file format is currently a binary file format based on the Microsoft Compound File Format. In Flash Pro CS5, the fla file format is a zip container of an XML-based project structure.

**.as** — .as files contain ActionScript source code in simple source files. FLA files can also contain Actionscript code directly, but separate external .as files often emerge for structural reasons, or to expose the code to versioning applications.

**.swf** — files are completed, compiled and published files that cannot be edited with Adobe Flash. 

[Description of Adobe all file formats on Wikipedia](http://en.wikipedia.org/wiki/SWF#Related_file_formats_and_extensions)

The best way to start with Flash is to learn the Adobe Flash CS (<http://www.adobe.com/products/flash/>).
swf is a flash package that can contain, among other things, source code.

You can find [free flash decompilers](http://www.google.com/search?q=swf%20decompiler) that will disassemble it for you.