I would like to create a new widget area to the left side of the content.

Does someone know in which file and where I should put the code?

And if someone knows a tutorial on how to do it that works with the Montezuma theme I'd be more than happy!

I just solved it all by myself! :D Very simple, just added the red code in under
Montezuma options/Main templates/index.php

<!--<div id="container">-->

<?php get_header(); ?>

<div id="main" class="row">
  
        <div id="widgetarea-left" class="col3">
		<?php dynamic_sidebar( 'Left Widget Area' ); ?>
	</div>

	<div id="content" class="cf col6">
		
		<?php bfa_content_nav( 'multinav1' ); ?>
		
		<?php bfa_loop( 'postformat' ); ?>
		
		<?php bfa_content_nav( 'multinav2' ); ?>
		
	</div>
	
	<div id="widgetarea-one" class="col3">
		<?php dynamic_sidebar( 'Widget Area ONE' ); ?>
	</div>

</div>
	
<?php get_footer(); ?>

<!--</div>-->