Q: How to use iTunes prepaid gift card

What problems are you having with it ? You should be able to redeem it on your computer's iTunes or on an iPhone, iPad or iPod Touch : redeeming a card.

If you are getting an error when trying to redeem then what is the error e.g. invalid code, card not activated, has some of the code been scratched off the card ?

If the card hasn't been activated then can you take it back to the store that you bought it from so that they can activate it properly ? If not 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

Similarly if you are getting an 'invalid code' or some of the code has been scratched off the card you can contact iTunes Support via the above link.

If it's a different problem ... ?