In Magento you can load your model with getModel. You can optionally pass parameters to the model class constructor. All you need to do is:
Mage::getModel('model_class', 'arguments_as_array_if_any');
For example: I want to load Data.php model from: https://github.com/dbashyal/Magento-General-Module-Extension
So, to load this model class i need to call as:
Mage::getModel('tgeneral');
As Data.php is default file name, you don't need to pass file name. But if you need to load something like Observer.php from model then you have to call it as:
Mage::getModel('tgeneral/observer');
If you have a construct function and want to pass arguments then you need to load model as:
Mage::getModel('tgeneral', array('key1' => 'value1','key2' => 'value2',...));
Src: Mage.php 'getModel()' and app\code\core\Mage\Core\Model\Config.php 'getModelInstance()'