Q: How can I know what is the code of my gift card

It should be on the card : how to redeem a card. If you've scratched it off and the page that you posted from doesn't help then you will need to try contacting iTunes Support (you will probably need to give them images of the front and back of the card, and possibly its receipt) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes