1. First, we must add the JSON API module to our Drupal site:

cd /path/to/drupal8
composer require drupal/jsonapi

2. Install the JSON API and Serialization module. Once the module is installed, the API endpoints will be active.:

3. To perform a request, you must pass an Accept header with the value application/vnd.api+json.
4. For the JSON API specification, every resource must have a unique type name, and JSON API derives this from the entity type and its bundle. The endpoint for retrieving Article nodes will be:

http://127.0.0.1:8888/jsonapi/node/article

5. The entire request can be executed with the following command:

curl -X GET \
  http://127.0.0.1:8888/jsonapi/node/article \
  -H 'accept: application/vnd.api+json'

6. The response will resemble the following. The content values will be in the attributes property:

{
  "data": [
    {
      "type": "node--article",
      "id": "c897acba-eb81-454a-94ed-13107fd205cf",
      "attributes": {...},
      "relationships": {...},
      "links": {
        "self": "http://127.0.0.1:8888/jsonapi/node/article/c897acba-eb81-454a-94ed-13107fd205cf"
      }
    }
  ],
  "links": {
    "self": "http://127.0.0.1:8888/jsonapi/node/article",
  }
}