Hi I have $15 Apple Store card how should I get to my phone please

If it's an Apple Store gift card, as opposed to an iTunes gift card or App Store card, then it can only be used in an Apple retail store or the Apple online store - you can't redeem it onto your iTunes account.

If it's an iTunes or app store card then you should be able to redeem it onto your account : http://support.apple.com/kb/HT1574