Prevent WordPress to encode html in post

how to prevent wordpress not to encode html in a post?

My problem is similar to this question. https://stackoverflow.com/questions/4990422/how-to-prevent-wordpress-to-encode-html-in-post

the right answer of that question is this.

<?php $content = htmlentities( html_entity_decode($content) ); ?>

This code will print HTML tags, and then decode HTML entities (like & to &).

but i dont know where to put this php code in wordpress. plz someone help me.