How to Authorize Computer to iPhone
Although you can connect your iPhone to your computer, you won't be able to play music, videos and other purchased content from your iPhone unless you authorize the computer through iTunes. Content that was purchased before January 2009 has digital rights management (DRM) protection, and in order to play the purchased content the computer must be authorized. Apple allows up to five authorized computers per Apple account to play purchased content.
Insert the wide, flat end of the USB cable into the iPhone's dock connector. Insert the flat, narrow end of the cable into your computer's USB port.
Click "Start," "All Programs," "Apple" and "iTunes" to run iTunes on a PC. For Macs, open the Applications folder in Finder and double-click the "iTunes" icon.
Click "Store" at the top of the menu and select "Authorize Computer" from the drop-down menu.
Type your Apple ID email address and password, and click the "Authorize" button.
Click "File" at the top of the menu and select "Transfer Purchases from iPhone."