The previous owner should have unpaired the Apple Watch, including removing Activation Lock from it, before selling their device.
If they have not yet done so, then ask the previous owner to follow the instructions here in order to prepare the watch for you to set up as your own, under your own Apple ID:
- Unpair your Apple Watch and iPhone - Apple Support
- About Activation Lock on your Apple Watch - Apple Support
You will then be able to set up the Apple Watch as your own device by following the instructions here (be sure to update your iPhone to the latest version of iOS first):
If you cannot arrange for the previous owner to do this, then you should, if possible, return the Apple Watch for a refund.
If Activation Lock is enabled, you will not otherwise be able to use the Apple Watch. There is no way around this security feature.