I got an iPhone 5 or 5s second hand and the previous owner doesn't know anything about the iCloud or password and is in activation lock mode help please!

The previous owner needs to log out of iCloud or go to https://www.icloud.com/find/ and erase the device, then remove it from their account. If they won't do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.     

See if they can recover the ID/password. Use every email address they can think of.

Apple ID - How to find your Apple    An iCloud e-mail address is an Apple ID.

Apple ID - Find              

If they forgot the Apple ID password, enter the Apple (iCloud) ID, and click Forgot Password. See if they can get a Rescue or Notification  email option if necessary.

Apple ID - Manage            

Apple ID – Forgot Password