I just answered the same question from another reader, so I copied it here:
Apple instituted Activation Lock to prevent lost and stolen iPhones/iPads/iPods from being used. As a result, you will need to have the original owner remove the device from his account before you can use it.
Once you contact the previous owner, they need to follow these steps to remove the iPhone from their account:
Go to Find My iPhone on iCloud.com.
If you don’t see Find My iPhone on iCloud.com, your account just has access to iCloud web-only features. To gain access to other iCloud features, set up iCloud on your iOS device or Mac.
1. Click All Devices, then select the device.
2. Click Erase [device], then enter your Apple ID password or your family member’s Apple ID password. Because the device isn’t lost, don’t enter a phone number or message.
If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.
If you erase your iPhone 6 or iPhone 6 Plus and you have credit and debit cards in Wallet for Apple Pay, Find My iPhone attempts to remove your cards immediately, even if your iPhone is offline. For more information, see the Apple Support article Use Wallet on your iPhone or iPod touch.
4. When the device is erased, click Remove from Account.