reset old iphone that someone else had. dont know there code

You'll need to reach out to the previous owner, and have them either physically sign out of the device or have them go to iCloud.com/find on a PC or Mac and click on "all devices" or "show devices", click on the device and remove.

If the original owner can't do that, they can do activation lock removal with the original receipt.

Please see this: How to remove Activation Lock - Apple Support

Only the owner of the iCloud account used to lock it can remove the activation lock.

Talk to the previous owner.