Automatically create a default set of posts for each new registered user

A combination of user_register and wp_insert_post should work fine:

add_action('user_register', 'create_postset');

function create_postset($user_id) {
    // userinfo
    $user_info = get_userdata($user_id);
    // Create post objects
    $post_1 = array(
        'post_title' => '1st post created for ' . $user_info->user_login,
        'post_content' => 'Lorem Ipsum',
        'post_status' => 'publish',
        'post_author' => $user_id
    );
    $post_2 = array(
        'post_title' => '2nd post created for ' . $user_info->user_login,
        'post_content' => 'Lorem Ipsum',
        'post_status' => 'publish',
        'post_author' => $user_id
    );

    // Insert posts into the database
    wp_insert_post($post_1);
    wp_insert_post($post_2);
}