i bought a second hand iphone but the icloud account the owner dsnt know the password and i need to put mine what do i have to do ??

Hello Ibrahiim.Ahmad,

Ask the previous owner of the iPad to Go to the following link and reset his Apple ID Password that he used to Activate Find My iPhone on the Device and once his password is reset ask him to Login to iCloud.com and then Remove the device from his iCloud Account.

Reset Apple ID Password : https://iforgot.apple.com/password/verify/appleid

Ask the Previous owner to follow this steps on iCloud.com after the password is reset :

If the previous owner is not present
Contact them and ask them to follow these steps to remove the device from their account:

1. Sign in to their iCloud account at www.icloud.com/find.
2. Choose the device from their Find My iPhone device list by clicking All Devices and selecting the correct device.
3. Click "Remove from Account" to remove the device from the account.

After the device has been removed from the previous owner’s account, turn it off by pressing and holding the Sleep/Wake button located on the top right side of the device. Then restart your device and proceed with device setup as you would normally.

and what happen if the previous owner cant be contacted?

excuse me sir with the new feature of the iPhone 6 how can i reach that person?

Contact them through the person you purchased the phone from. If they cannot assist you in that, then return the phone for a refund. Without the previous owner the phone is useless to you, and you will not be able to activate it. Apple cannot assist you in contacting the previous owner either.