The Model 
First declare an attribute to store the file name in the model class (either a form model or an active record model). Also declare a file validation rule for this attribute to ensure a file is uploaded with specific extension name.

class Item extends CActiveRecord
{
    public $image;
    // ... other attributes
 
    public function rules()
    {
        return array(
            array('image', 'file', 'types'=>'jpg, gif, png'),
        );
    }
}
You can add others validation parameters as described in CFileValidator. For instance, one can add a "maxSize" restriction (the PHP ini settings will of course prevail).

The Controller 
Then, in the controller class define an action method to render the form and collect user-submitted data.

class ItemController extends CController
{
    public function actionCreate()
    {
        $model=new Item;
        if(isset($_POST['Item']))
        {
            $model->attributes=$_POST['Item'];
            $model->image=CUploadedFile::getInstance($model,'image');
            if($model->save())
            {
                $model->image->saveAs('path/to/localFile');
                // redirect to success page
            }
        }
        $this->render('create', array('model'=>$model));
    }
}
CUploadedFile::saveAs() in one of the methods of CUploadedFile. You can also access directly to the file through its "tempName" property.

The View 
Finally, create the action view and generate a file upload field.

$form = $this->beginWidget(
    'CActiveForm',
    array(
        'id' => 'upload-form',
        'enableAjaxValidation' => false,
        'htmlOptions' => array('enctype' => 'multipart/form-data'),
    )
);
// ...
echo $form->labelEx($model, 'image');
echo $form->fileField($model, 'image');
echo $form->error($model, 'image');
// ...
$this->endWidget();
Another syntax is to use static calls in CHtml instead of CActiveForm. The result is the same as above.

<?php echo CHtml::form('','post',array('enctype'=>'multipart/form-data')); ?>
...
<?php echo CHtml::activeFileField($model, 'image'); ?>
...
<?php echo CHtml::endForm(); ?>