When you want to get the API representation of content, you need to make a GET request to the appropriate resource. For example, if you have a node with an NID of 1, you would make a GET request to /node/1?_format=hal_json. You will need to provide some authentication parameters unless you have changed your permissions to allow anonymous access to GET requests for nodes. To test this with cURL, you would make the following request:

curl --request GET --user myusername:mypassword --header 'Accept: application/hal+json' http://localhost/node/1?_format=hal_json 
 

The response will contain all the fields for the node, along with the links to the author of the node and entity bundle.