A proper noun is a specific name for a person, place, or thing. So, "Central Park" is a proper noun because it's the specific name of a park in New York City.
"Park" on its own is a common noun, referring to a general type of place.
To find the proper noun of a specific park, you need to know its name. For example:
* Central Park
* Golden Gate Park
* Hyde Park
* Disneyland Park