Q: how can use an itunes card to pay the music app?

Assuming that you mean subscribing to Apple Music, you shouldn't need to do anything special for it to use your account's balance, it should do so automatically : Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support

*You can use store credit to pay for your Apple Music membership, but you must have enough store credit to cover the amount of your first renewal. Learn how to add store credit to your account.

If you mean just buying music from the store, then just tap on the price button of the music that you want to buy