by Hubert Chathi.
1. In theory, your file could be anywhere on the server (as long as it's web-accessible). In practice, you want to put your file a) somewhere where it won't get lost (i.e. if you need to clone your install, you'll remember to copy that file) and b) somewhere that makes sense (e.g. putting it in the mod directory doesn't make sense). Creating a local plugin as Howard suggests is a good option.
2. You want to include the config.php in the main Moodle directory. So, yes, if your page is moodle/pages/contactus.php (assuming that moodle/ is the root of your Moodle installation), then you would "require_once('../config.php');"