i want change my i tune region, when i search any item then error come (this item you have request is not currently available in US store), when i sign in my account, then error come (i tune could not connect & an error 11556 is come)

We are fellow users here on these forums, you're not talking to iTunes Support nor Apple.

From http://support.apple.com/kb/TS3297 :

"Error 11556"

This alert occurs when iTunes is trying to do something not allowed by the iTunes Store in that country.

The most common example of this is trying to sign in while limited to iTunes U by Parental Controls.

For more information, see the steps from iTunes 9: "One Moment Please" or "Error (-50)" message when accessing iTunes Store.

What country are you in ? You can only use the iTunes store for the country where you are, you can't buy/download content from another country's store

Have you checked what I included in the my above post e.g you've checked the 'parental' tab in iTunes > Preferences on your computer's iTunes ?