Not enough access privileges for iTunes.

Whenever I open iTunes it gives me a message: 'You do not have enough access privileges for this operation.' And whenever I try to buy something it gives me this message: 'iTunes couldn't download your purchace; You do not have enough write access for your iTunes Media Folder or a folder within it. Change permissions (in the Finder) and then choose Store > Check for Available Downloads.' I do everything it tells me to do and it just will not work. If anyone knows how to solve this issue please help. Thank you.