I got a second hand iphone and I do not know how to unlock it

There's multiple kinds of "locking" on devices. Click on the links below to find instructions on how to deal with your specific situation:

1) iCloud: Activation Lock - Activation Lock for iPhone, iPad, and iPod touch - Apple Support - This is a security feature so only legitimate (e.g., not stolen) use of devices is possible. Read this support article about how to remove Activation Lock --> How to remove Activation Lock - Apple Support - 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. If you are the owner, see the part at the end about asking Apple to reset the device.

2) Device lock: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled, read this support article if you are using iOS or iPadOS 15.2 or higher --> If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support or here if you do not meet the criteria for the previous method--> If you forgot your iPhone passcode - Apple Support

3) Carrier lock: Read this article about how to unlock your iPhone for use with a different carrier. --> How to unlock your iPhone for use with a different carrier - Apple Support Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.