My Itunes won't let me buy apps or songs, and I can't even download the free ones

Hello. I have an iphone 5. Iv'e had it scince May of 2013 (this year). I really haven't had any problems before with Itunes, but I can't buy songs or apps, and I cannot even download the free apps! I have recently got an Itunes gift card for Christmas, and I put it into my Iphone, and Itunes says I have the money from the gift card. I started to notice this when I was my grandparent's house a few days ago for Christmas, and it wouldn't let me get an app, even though it was free. I thought it was because of a diffent type of WiFi or because I was at a different place, but when I got home, it still wasn't working. Whenever I try to buy or download something, it says, "Verification Info." I click continue, and it says that I need to buy money with a credt card, even though I have money in my account! This has also happend to my sister (she has an iphone 4). Please help!