REST API HELP

Dear All,
Im Trying to hit a Get Request from my https server
but it always Return NULL Please help,

https://tierion.com/docs/dataapi#api-create-datastore

Code:

$service_url = 'https://api.tierion.com/v1/datastores';
   $curl = curl_init($service_url);
   $curl_post_data = array(
        "X-Username" => 'EMAIL ADDRESS',
        "X-Api-Key" => 'API KEY',
        );
   curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
   curl_setopt($curl, CURLOPT_POST, true);
   curl_setopt($curl, CURLOPT_POSTFIELDS, $curl_post_data);
   $curl_response = curl_exec($curl);
   curl_close($curl);

   $decoded = json_decode($curl_response);
if (isset($decoded->response->status) && $decoded->response->status == 'ERROR') {
    die('error occured: ' . $decoded->response->errormessage);
}
echo 'response ok!';
var_export($decoded->response);

?>

Here is my potato in advance...