How to Download iTunes to iPhone 3G
The iPhone 3G allows you to download music and music videos directly from the iTunes Store app on your iPhone. Due to the heavy data use for larger files such as movies and TV shows, you'll need to have Wi-Fi enabled on your iPhone 3G before you can download these items directly to your iPhone. Purchasing and downloading media from iTunes requires an active iTunes account that has a credit or debit card attached to it. Alternatively, you can also sync your iPhone 3G with the iTunes program on your computer via a USB cable.
Download iTunes from iTunes Store App
Press the "Home" button at the bottom of your iPhone 3G's screen. Unlock the device by sliding your finger across the arrow bar.
Tap the "iTunes" icon on the screen and tap the "Music" icon at the bottom of the screen.
Tap "New Releases," "Top Ten" or "Genres" to search by a specific category. Tap the "Search" icon at the bottom of the screen to search by song title or artist.
Tap the "Purchase" button and tap the "Buy Album" or "Buy Song" button. Type your iTunes account password and tap "OK" to complete the purchase.
Sync iTunes to iPhone using Computer
Click "Start" on the Windows and click the iTunes program. Alternatively, click the iTunes icon on your Mac's screen dock to open the application.
Plug the iPhone USB cable into the bottom of your iPhone, then plug the other end into your computer's USB port.
Click the iPhone tab in the iTunes sidebar and click the "Music" tab. Check the box next to "Sync Music" and click the "Apply" button.