Leonardo,
Unfortunately "How to build bar code readers" is outside the scope of this forum. I found a wealth of information by doing a basic search on popular search engines.
I would discourage re-inventing the scanning mechanism. There's almost certainly an off-the-shelf scanner out there that will be cheaper than starting from scratch. Regardless, once you have your barcode string, you will find plenty of HID examples in our Tiva-Ware release that illustrates how to create HID packets and send them to the host.
Good luck!
--Miles