As first step i would suggest registering to the forums
Then, I would suggest to play with your C, work with reading / writing binary files (anything you want) to see how it works, view file with some hex-viewer to see how does it look like 'inside'.
Then take a look at some documented file format, lets say .BMP picture. You can find its description (and many more) on
www.wotsit.org. Take a look at BMP description and try to figure it out by looking through hex-viewer into some .BMP file. Try changing values inside file and stuff, try everything. Then you can try to write an file reader which reads information from BMP. This way you can earn some experience, and then to go to undocumented file formats like game graphics.
cheers and good luck
--edit--
oh i forgot to mention that it is a very long way to decode some file format ...
