Why won't Click and Buy work with ITunes?

This is sooo frustrating! I have tried so many times to buy ITunes songs via Click and Buy. I have managed to set up an account but each time I get re-directed to ITunes it goes back to the beginning of the process and tells me I have already authorised it. I can't seem to be able to get beyond this point. Please can someone help and explain it in plain English as I am not the most technologically minded of people! Thanks!