Geting started with models

[eluser]Unknown[/eluser]
Hi all, I use CodeIgniter since two or three days and I don't understand how I should name models, plus their filenames. I tried what I understood from the user guide but I got errors.

Here is my code:
application/controllers/computation.php
http://pastebin.com/7qDLY4bg

application/models/computation.php
http://pastebin.com/jrUUarX1

But I got these errors:

<h4>A PHP Error was encountered</h4>
<p>Severity: Notice</p>
<p>Message: Undefined property: Computation::$Computationmodel</p>
<p>Filename: controllers/computation.php</p>
<p>Line Number: 28</p>

PHP Fatal error: Call to a member function forum() on a non-object in F:\www\jvfstats\application\controllers\computation.php on line 28

Could you tell me what is wrong please?