I currently created an extension which does a series of works to the category and product url.

Then I need to tell Magento not to include some of the categories and products if they have certain attributes.

But I'm a bit new so I need to know where do I setup my function. Is it in Observer.php or do I need to create another observer?

How to hook the event to my function. I think that would be in the config.xml like below:

<frontend>
    <events>
        <core_block_abstract_prepare_layout_before>
            <observers>
                <updater>
                    <type>model</type>
                    <class>My_Updater_Model_Observer</class>
                    <method>beforeLoadLayout</method>
                </updater>
            </observers>
        </core_block_abstract_prepare_layout_before>
    </events>
</frontend>

Thanks.