Just Paste this code on your header.php

<meta itemprop="name" content="<?php esc_attr( the_title() ); ?>">
<meta itemprop="description" content="<?php echo esc_attr( get_the_excerpt() ); ?>">
<meta itemprop="image" content="<?php $src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), array(300, 300), false, ''); echo $src[0]; ?>" >