How to get the pixel data so as to work on it?

Hi all,

I am trying to write a plug-in that will crop and scale the image, edit the selection in the image document based on image data. For this purpose I need the pixel data of the image. How to get the image data as different pixels and what is the data structure used to store this data?

Assumption: No layers i.e., only background

Thanks in advance,
Sailu