How to unlock my sprint iPhone?

What do you mean by "unlock"? There's 3 kinds of device locking.

1) Carrier lock: How to unlock your iPhone for use with a different carrier - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

2) iCloud: Activation Lock - http://support.apple.com/kb/PH13695 - This is a security feature so only legitimate (i.e., not stolen) redistribution of devices is possible. See: Turn off Find My iPhone Activation Lock - https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it. Get them to remove it or return the item for a refund.

3) Device lock: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - https://support.apple.com/HT204306

What type of lock - forgot passcode? Activation lock? Carrier lock?

You contact Sprint and go through their process to see whether you are eligible for an unlock.