Assuming that you mean iTunes gift card, then if you haven't already done so then you can redeem it onto your account : http://support.apple.com/kb/HT1574
As long as you are buying content for yourself then that should automatically be used for your purchases - click on an item's price to buy it.
How purchases are billed : http://support.apple.com/kb/HT5582
When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.
If you are wanting to gift items, as per the HT2736 page that you posted from, then you will need to have a credit card on your account - you can't use your account's balance for gifting, that can only be used to buy content for yourself.