How to download free apps in iPhone 4s every time it ask to purchase even on free apps

I have bought new iPhone 4s and I am trying to download some free apps on my iPhone but whenever I try

To install a free app first it ask for my I'd and password which I enter and then it ask for I'd and password again

To pay for a free app

But why shall I pay it's free

Every time same problem occurs whenever I try to download any free app

Please help