I need to find the Host URL, Site key and API key
Host URL

Depending on the context of your situation, this is probably referring to the domain name of the site where you use CiviCRM. So if you see https://example.org/civicrm when using CiviCRM, then the host URL is example.org

Site key

This is one value, used globally for your CiviCRM installation and stored in the civicrm.settings.php file on your server. (The location of this file will differ depending on what CMS you use.)

Within the file, you'll find a section like this:

if (!defined('CIVICRM_SITE_KEY')) {
  define( 'CIVICRM_SITE_KEY', 'J8UdnKcpN93x0mvv');
}


So here, your site key would be J8UdnKcpN93x0mvv.

API key

API keys are unique to CiviCRM contacts. You'll need to create an API key for a contact if you want to use the API through the REST interface.

If you are setting up an external service to connect to your CiviCRM installation, it's probably worth creating a new contact record specifically for this service to interact with your site. Then you can grant specific permissions to this user just like you would do for a real person.