I cant download applications from app store iPhone 3G

when i want to download any application from the app store an error occurs..

after pressing the "install app" button, the icon of the app appears on the springbord with "waiting" written below it (as if its going to start downloading), but suddenly a messege appears saying (unable to download application      "app's name" will be avaliable for download when you log in to the iTunes Store on your computer) and even when I download apps from computer and want to transfer them to iphone using automatic sharing, same problem occurs (and the automatic sharing is turned on)

How can I fix this error??