Need help please, upgrade iOS iPhone 3G

first I've :apple: Iphone 3G version 3.1.2 7D11

when I connect to computer (mac or windows) with USB, it just show sharing image..

when I wanna Upgrade my IOS version, I need iTunes got shared on my mac..

any one have a clue to solve my problems?
to show iTunes on mac or windows when my iPhone connect to computer..

please help me