Get the_content with ajax

I’m using WordPress (WP) as the backend for an app that delivers content on a map.

I’ve been trying for a couple of days to get my head around AJAx in WP and have found a good tutorial I’m trying to follow.

I’ve adapted it for use in the theme, however how do I get it to ‘GET’ the_content?

The parts that are relevant:

function the_content( $content ) {

global $post;

if( is_single() && 'post' == $post->post_type ) { //I'll want post from a specific category, not just single posts.

$content .= '<button class="ajax-yall" data-postid="'. $post->ID .'">'.     __('Click me!', 'ajax-yall') .'</button>'; //here I want it to get the post content of posts in a particular category
}
return $content;
}

and

$.get(
        ajaxYall.ajaxurl,
        {
            action  : ajaxYall.action,
            nonce   : ajaxYall.nonce,
            postID  : $el.attr('data-postid')
        },

I’ve had various attempts at this, but my knowledge on the subject isn’t sufficient.