when i want to start iTunes it is showing the message 'The folder iTunes is on a locked disk or you don't have write permissions for this folder.' and it quits after clicking ok.
You don't give any of the back story? Did it work properly before?
Did this just start happening?
New Machine ?
Transfer files out to a external HD?
All I can say at this point is (command-i) with the app highlited in your apps folder, In this info pain, unlock the pad lock lower right and give yourself permissions.