i want to customize the build in portlet mainly shopping portlet ...
can any one help me plzz

Help in the sense of developing the customization for you?
Or
Help in the sense of guiding you with how to start about it?

Anyways, I prefer the second option so since its a pretty generic question, here is a generic answer:
1) If you just want to customize the look and feel, services and the JSPs, then plugin-hook is what you need.
2) If you want to customize or override Action classes and other stuff which you can't do with a hook then plugin-EXT is what you will need.

Hope it helps.

Just download Liferay's source code from the download page and customize away! Select "Portal Source" from the drop down list.

http://www.liferay.com/downloads/liferay-portal/available-releases