I copied this from the iTunes: Syncing media content to iPod kb article. It may help to see the screenshots which are on the website, but I did not copy here. The website is posted at the bottom of my response.
This applies to the iPod, iPhone or iPad. If you are using iTunes 11 - enable the sidebar by going to View>Show Sidebar from the iTunes menu at the top.
If you want iTunes to update your music, videos, or podcasts whenever you connect your device, sync automatically. Whenever iTunes is open and your device is connected using the USB to Dock Connector cable, iTunes will sync.
- Connect your iPod to your computer and open iTunes.
- Select your your iPod in the Devices section on the left side of the iTunes window.
- In the main iTunes browser window, select Music.
- Select the option to Sync Music. You may also select "Include Music videos" if you would like to sync Music videos to a device.
- Your device will need to have enough free space to sync the content that you have selected from your library.
Tip: You can use the capacity bar at the bottom of the device-settings window to determine how much free space is available. You can determine content size by selecting it and looking at the bottom of the iTunes window.
- Click Apply and iTunes will begin syncing the audio content you've selected to your device. The iTunes LCD will display the sync progress and notify you when the sync is complete.
- To disconnect your device, select it under Devices and click the eject icon next to it.
This is the website