Messages emerging every time I click on buy!!Help

[antia]

Hello to you all!

I am a new user, considering that i have just bought an iPhone 4s 2 days ago. I don't know as much as you do, but I m trying every moment passing by to learn.

I want to download an app that it isn't free and I just can't.
, saying:
You apple id is incorrect (which it isn't for sure)
For buying E-wallet you must enter a valid apple id (which I have, every time)
And at the end, two choices one saying OK and the other saying, Enter a Valid Apple id

My question is:
How can I download this app? I have tied many times and I couldn't. What am I doing wrong?
And I honestly need it because it has to do with a simple calculation of my income-outcome.

Therefore I need it!
Can someone please help me?

 

[office politics]

are you trying to buy using you phone or itunes on a computer?

on the phone, you may be able to goto settings and logout and log back in and try again.

you should also be able to buy apps via itunes on the computer

like this -> How to Purchase iPhone Applications | eHow.com