Q: i have a $13 credit on y itune acct.  how do i put it towards my purchase?

Hello trishdowney,

Thanks for using Apple Support Communities.

For more information, take a look at:

Why can’t I select None when I edit my payment information?

http://support.apple.com/kb/ts5366

When you sign in with an existing Apple ID (that you haven't used in the store), you must provide a payment method. If you want to remove the payment method after you create the account, you can change your payment information to None.

iTunes Store: How iTunes Store purchases are billed

http://support.apple.com/kb/HT5582

Order of iTunes billing methods

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.

Have a nice day,

Mario