How do you purchase an app without using a credit card?

I am new in Apple products and the iPod Touch. I've selected none for my credit card because I don't have one. How do I purchase something WITHOUT a credit card? Please give me a detailed tutorial. Thank you all very much for your noobie helping effort! =]|||Using a gift card|||Debit card, credit card, gift card, or PayPal.|||The only really way to do this is to buy a gift card from a music shop and use that!