If it's your phone, enter the Apple ID username and password that was first used to activate it. Your iPhone will open. If you forgot your Apple ID or password, you will need to factory reset it.
There are many online services available to help you remove the iCloud lock from your iPhone without using any tools. You will need provide the site your device details and make a payment.
If you bought a used iPhone and the deivce is iCloud locked, you can contact the previous owner and aks her/him to remotely remove the Apple ID and erase the device. Just go to iCloud official site and sign in with their Apple ID and select Find iPhone. Select All Devices and select the iPhone that is iCloud locked, then choose Erase.