Wrap posts p tags in div

You need to add it to a JS file and then enqueue it in your functions.php

jQuery(document).ready(function($){
  $('p').wrap('<div class="post-txt" />');
});

If you already have a js file, you can add it to that file and you’re done.

Else, save the above snippet in a new file. Put that file in a folder called js in your theme folder(for the sake of clean code). Let’s say you called that file wrap.js.

Add the following in your functions.php file:

function wpse_134325_scripts() {
    wp_enqueue_script( 'wrapper', get_template_directory_uri() . '/js/wrap.js', array('jquery') );
}

add_action( 'wp_enqueue_scripts', 'wpse_134325_scripts' );

You can read more about enqueing scripts on the Codex.