Thank you very much for answering so fast guys; for some bizarre reason, this seems to be the only of my questions that is being answered here in the forums.
So, esmi, I had already read that wordpress tutorial about the 404 error template; I'm using a twenty eleven child theme which already comes with a 404 Template file (404.php). Anyway, I read that tutorial again and I can't find the piece of code it gives:
<?php get_header(); ?>
<div id="content" class="narrowcolumn">
<h2 class="center">Error 404 - Not Found</h2>
<?php get_sidebar(); ?>
<?php get_footer(); ?>
Besides, I must confess that it really seems extremely odd to me the fact that the default twenty eleven 404 error page is that clumsy blank page with a text. I must also point that my knowledge regarding code editing is extremely poor, so it would take a lot of time for me to create a nice 404 error page out of nowhere, just by writing the code, which is exactly why I've been searching for a plugin to do so.
Well, any ideas?
Thank you very much for your help!