Use the following jQuery . You will need to have a div with the myDivClass class, since jQuery uses CSS selectors to search for elements. Part of document.ready is to make sure your page is fully loaded / parsed before Javascript is executed (this is an important step).
$(document).ready(function() { $('.myDivClass').slideDown(); });
Here is an JSFiddle as an example to slide the slide down on the button.
PS If you use Firebug or Chrome correctly, you can try it right now on this page!
$('#hlogo').hide().slideDown('slow');
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