I don't have an iOS device, but I do use iTunes on my Mac and Windows desktops. About a month ago, I wanted to rent a movie on iTunes (as a test), but my iTunes balance was $0, so I tried to figure out how to tell iTunes my credit card information. I couldn't find any way to do that. Maybe it's not possible? It was frustrating but my solution was just to buy a $15 iTunes gift card when I was grocery shopping at Safeway.
After reading this story, I'm glad I couldn't give my credit card information to iTunes.
You probably are even less interested in adding your credit card, but if you want, there's a "Payment Information" section if you click on the "Account" button dropdown by your apple id in iTunes.
I just changed my password to one more unique (I was reusing it elsewhere), and finding the place to do so was surprisingly hard to find (IMO, it's harder than adding payment information).
After reading this story, I'm glad I couldn't give my credit card information to iTunes.