How to use a custom class in wordpress

Hi!

Ich have class that has to be build and printed (<?php echo $sb_bar ?>) within a theme:

require (‘inc/bar.class.php’);
$sb=new Bar();
$sb_bar = $sb->Render();

Where can I build the class?

When I include

$sb=new Bar();
$sb_bar = $sb->Render();

within a theme, wordpress stops rendering.

Thanks!