Syncing and arranging applications from the App Store – You can purchase and download applications for iPhone and iPod touch from the iTunes Store using iTunes on your Mac or PC or using the App Store1 on iPhone or iPod touch. If you are unable to sync applications, or unable to arrange applications on your iPhone or iPod touch screen, please verify that you have followed the steps below. For additional information about using your iPhone or iPod touch, please see the User’s Guide available from apple.com/support/manuals. For additional information about the App Store and applications, please see the App Store FAQ.

http://support.apple.com/kb/HT1977

Troubleshooting applications purchased from the App Store – Try the following troubleshooting steps on your iPhone or iPod touch in order to resolve an application’s issue. 
 

http://support.apple.com/kb/TS1702

How to determine if the iTunes Store is available in your country – The iTunes Store is available in many countries. You can determine if the iTunes Store is available in your country by following the steps outlined in this article.

http://support.apple.com/kb/HT1924

iPhone and iPod: How to rename your iPhone or iPod – To change what your iPhone or iPod is called when it is displayed in iTunes, follow the steps below.

http://support.apple.com/kb/HT3965

Update and restore alert messages on iPhone and iPod touch – When you are trying to update or restore your iPhone or iPod touch with iTunes, the update or restore process may stop, and you may see an alert message in iTunes. You may also see the "Connect to iTunes" screen on your iPhone or iPod touch:

The alert message in iTunes may also include one of the following numbers (but is not limited to): 2, 4, 5, 6, 9, 13, 14, -18, 19, 20, 21, 23, 28, -48, -50, 1002, 1011, 1013, 1015, 1601, 1602, 1603, 1604, 1608, 1609, 1611, 1612, 1639, 2009, 3004, 3013, 3014, 3015, 9807, -9808, 9844.

http://support.apple.com/kb/TS1275

iTunes Store: How to redeem an iTunes Gift Card – If you have an iTunes Gift Card that you want to redeem in the iTunes Store, follow the appropriate tutorial below.  If you want to learn how to redeem an iTunes Store online Gift Certificate, refer to iTunes Store: How to use your iTunes gift certificate.
Note: Some features or services are not available in some iTunes Store countries.
 

http://support.apple.com/kb/HT1574

Removing and reinstalling iTunes, QuickTime, and other software components for Windows Vista or Windows 7 – Learn how to cleanly uninstall iTunes, QuickTime, and other essential software components from your Windows Vista or Windows 7 PC; (for a Windows XP or Windows 2000 PC, refer to Removing and Reinstalling iTunes, QuickTime, and other software components for Windows XP).

http://support.apple.com/kb/HT1923

iTunes for Windows XP: Troubleshooting unexpected quits, freezes, or launch issues – This article provides some general suggestions when trying to troubleshoot an issue where iTunes for Windows won’t open or unexpectedly quits. It may be useful to print this document before following the steps outlined below.
Learn about troubleshooting iTunes for Windows in Windows Vista or Windows 7.
If you are having trouble installing iTunes, go here for help. However, if you see one of the following specific error messages, click on the corresponding link for more information.

The iTunes application could not be opened. An unknown error occurred (0×666D743F)
iTunes 7 for Windows: iTunes had detected an audio configuration problem

http://support.apple.com/kb/TS1421

iTunes for Windows Vista or Windows 7: Troubleshooting unexpected quits, freezes, or launch issues – This article provides some general suggestions when trying to troubleshoot an issue where iTunes for Windows Vista or Windows 7 won’t launch or unexpectedly quits or freezes. It may be useful to print this document before following the steps outlined below.
For a discussion of troubleshooting iTunes for Windows in Windows XP, see iTunes for Windows XP: Troubleshooting unexpected quits, freezes, or launch issues.
If you are having trouble installing iTunes, see Trouble installing iTunes or QuickTime for Windows.

http://support.apple.com/kb/TS1717

How much content will fit on my iPod or iPhone? – Several factors affect how much content will fit on an iPod or iPhone.

http://support.apple.com/kb/HT1867