I had written a small shell script which can format usb drive , (default vfat) , but i need it to get started through context menu .
Is there any way i can do it ..?

Well, probably. But could you provide a little more information about the context in which you are making a reference to a "context menu?" Are you, perhaps, referring to the GNOME "Applications" menu? Or do you just want to have a button for the application on the GNOME panel? (The one that, by default, is across the top of the screen although you may have moved it.) Or, you might be using some other window manager (KDE, etc.).

If you want to add it to the GNOME "Applications" menu, install the menu editor and edit the menu. If you just want to add something the the "top" panel, right click on it and select the appropriate "Add a program" tool. And so on for the other desktops.

But without more context to your question, more detailed answers are hard.

(Sorry about the "context" comments, but the pun was irresistible.)

Yea , normally other OS , like Windows had an option when rightclicked any of the drive , it will show in that menu , "Format"
Understood right .. ANy way thanks for the info u had given ..

Ah, I see. What you're looking for is the pop-up menu definitions. For that you find me at a loss - I never permit icons on my desktop, so I've never looked for such menus.

If I understand the freedesktop stuff correctly, you need to locate the correct <something>.desktop file, get to source code for the referenced application, and modify it to do what you want. But, as I said, I've never bothered to use desktop icons, so, obviously, I've never tried to modify them. (If you do have an icon on your desktop, a ls ~/Desktop should show you the name of the .desktop file being used.)