Q: Buying itunes with a voucher

If you haven't already redeemed it onto your account then there are instructions on this page for how to do so : http://support.apple.com/kb/HT1574

To buy an item, find the item in the store and click on its price to buy it. As long as you are buying content for yourself (you can't gift content via your account's balance) then your balance should be used up first - from 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 it's a new account, then depending upon how you created it you might be asked to enter your credit card details before the account can be used to download any item from the store.