As Mars20 correctly noted, Apple Watch works in conjunction with an iPhone. The interwork is industry best, by far imho, but the definite need for iPhone is the sticking point.
Make sure the owner who gave it to you has done their due diligence and turned off activation lock. They can do that by going into their iPhone watch app to erase it and disable activation lock, or go into their iCloud account->FindMy. Without Activation Lock removed, the Watch has no functionality whatsoever and never will.