To use the API properly you will need the API Token. API Token is a form of identifying the user through API. To get the API Token you will need to log into your account and go here. And click Recreate Token and the string of the form: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx is your API Token. Afterwards you can use it in terminal commands directly or via a file, more about this here.
To use API you will need the knowledge of the json file composition and names of metadata fields. Afterwards you can learn the basics of the API usage.
Images below show how to get from the main page to the API Token page and how to create or recreate the API Token.
