How to call external api to pull data into json format?
It depends on what you wish to do with said data.

If you are looking for a purely code approach, you could do something like:

$response = drupal_http_request($url);
$result = json_decode($response-&gt;data);


If you want a Rules based approach, you could could use the Rules HTTP Client module, which allows you to do essentially the same as the above code but via the Rules UI.