Get my site session in wordpress?

WordPress doesn’t use PHP session ($_SESSION). If you want to use it you need to do at your own. For example:

add_action('init', 'wpse_session_start', 1);
function wpse_session_start() {
    if(!session_id()) {
        session_start();
    }
}

But, if you want is to get the user id of the current user, you can use get_current_user_id():

<?php $user_ID = get_current_user_id(); ?>