If you want to server all your Moodle pages with SSL (instead of just the login page), simply set $CFG->wwwroot in config.php to use the HTTPS URL, like:
$CFG->wwwroot = 'https://your.moodle.site';
But if I want to use only few Moodle pages of my site with SSL, then what should I do??
I'm afraid that's not a supported configuration. Right now it's either login page only, or the whole Moodle site.