simple_ajax.php

//prepare javascript
function ajax_enqueue()
{
	wp_enqueue_script( 'my-script',get_template_directory_uri() . '/js/example.js', array(), '1.0.0', true );

	wp_localize_script( 'my-script', 'myAjax', array( 'ajaxurl' => admin_url( 'admin-ajax.php' )));        
}

add_action('wp_enqueue_scripts', 'ajax_enqueue');


//to enable ajax call on jquery
function ajax_sample()
{
	$sample_ajax     =     $_POST['hello'];

  	echo $sample_ajax;

	wp_die();
}

add_action('wp_ajax_ajax_sample','ajax_sample');
add_action('wp_ajax_nopriv_ajax_sample','ajax_sample');

?>


<script type="text/javascript">
	
	jQuery(document).ready(function($) {
    'use strict';

    $(".btn_submit").click(function(e){
        e.preventDefault();

        var hello = $('.txt_input').val();

        $.ajax({
            type:"POST",
            data:{
                action:'ajax_sample',
                hello:hello,
            },
         	  url : myAjax.ajaxurl,
            success:function(data)
            {
                alert(data);
            }
         })
     });
 });
</script>