$yourModel = Mage::getModel('yourmodulename/yourmodelname')->load($id);

$return = $yourModel->abc();

$yourModel = Mage::getSingleton('yourmodulename/yourmodelname')->load($id);

$return = $yourModel->abc();

Mage::getModel('modulename/modulename')->yourMethod();

Mage::getModel('catalog/category')->load($id);

<models>
        <modulename> <!-- <-- this here is the first part of your getModel() call -->
            <class>Packagename_Modulename_Model</class>
            <resourceModel>modulename_mysql4</resourceModel>
        </modulename>
        <modulename_mysql4>
            <class>Packagename_Modulename_Model_Mysql4</class>
            <entities>
                <abc>
                    <table>table_abc</table>
                </abc>
            </entities>
        </modulename_mysql4>
    </models>

Mage::getModel('modulename/abc')->yourMethod();