Showing HTML is so simple. It's so simple you don't even need a tutorial. Just an ISO SPECIAL CHARACTERS (LATIN BASED) chart. In HTML this sign < starts an HTML tag. Meaning that when you type this, your webbrowser takes it as a tag. And takes on that tags actions and therefor the tag that was supposed to be shown is by default invisible. So all you need to do is this:

Trade every < sign in the code you are trying to show with &lt;
and
Trade every > sign in the code you are trying to show with &gt;