Connect the iPod to your computer. iTunes should automatically recognize it and display it in the left-side "Devices" panel.
Wait for a few moments. If the iPod is registered, a dialogue box will pop up in the middle of your iTunes application screen telling you that it has been registered to another machine. If this is the case, iTunes will prompt you to erase the iPod and sync it to your iTunes.
The iPod has been around long enough now--and has been through enough generations--that it is not uncommon for people to purchase second-hand or even third-hand models of Apple's MP3 player. iTunes's limitations on the amount of computers an iPod can be used on can cause frustration and confusion, so it's best to find out whether the iPod you bought from your friend is already registered.
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