what about if i dont know who is the previous owner?

See Turn off Activation Lock https://support.apple.com/en-us/HT201441. If you acquired your device second hand from an individual (almost invariably not the prior legal owner), you will need to obtain the needed password from the prior legal owner (almost invariably not the seller) or have the prior legal owner sign out of iCloud; erase the device; and remove it from their device list. If you cannot do so, then the device is useless to you.