Pradeep, we understand what you want to do! To implement what you want to do from first principles is a lengthy process. As the others have mentioned, you need a cpu that has USB and/or USB OTG hardware. Then you need the software to actually talk the required USB protocol THEN you need filesystem code to read/write the data in the required format. This is all done in the Vinculum product to make your life easier. OR you can look at the various products from Atmel that have USB hardware built in then you only have to worry about getting the USB and filesystem code going (Atmel have examples). Otherwise there is always the SD/MMC cards that are much simpler to interface and forgoes the issue of special USB hardware for your cpu.

Whilst a pen drive may look cheap and simple, there is a lot of work that goes on behind the scenes to make it that way!