iTunes mess

The iTunes on my previous iMac still had my old email address, and updating it as a constant challege with the old password, so I decided to delete it and download it again with the new address. It wouldn't allow me to delete, as "...iTunes helper was in use", so , having failed to identify what or where iTunes helper was, I tried to delete the whole thing from the list in the hard drive, and couldn't do that either. Now I can't open it or delete it, so am in a pickle.

Can anyone walk me through a process to remove iTunes from the computer? All items have been saved.

Thanks in advance

Nick