Download free apps with no money on my debit card, why does it say free, but need money on your card, help me here please!

Need help with free apps, that you cant download without money on your credit card, mine is a debit card, and the only time I can download fee apps is when I have money on my card, so these aren't free, so help me here, tired of trying to download free apps, also got an iTunes gift card, and what are they actually used for?