Can't purchase apps from the Store

Hello,

I am trying to purchase some apps from my Iphone. But the system is not allowing me. I keep getting this error in iTunes: "for assistance contact itunes support at www.apple.com/support/itunes/ww/". Also, on my phone I am getting this error: "please contact itunes support to complete this transaction". What should I do now?

Thank you

[Re-Titled by Moderator]