SP.js file not loaded in a simple JSOM example

I am creating a simple example that demonstrates client object model using javascript. I have ceated a simple JS file in Visual Studio and an aspx page using the SharePoint Designer.

I have used the following command in my aspx page to include the sp.js and sp.runtime.js files but no success and so I get an error which says "ReferenceError: Type is not defined" when I run the page. the command that is used to include the sp.js is:

<script type="text/javascript" src="/_layouts/sp.js"></script>

and the code in my js file which I created using Visual Studio is:

jQuery(document).ready(function ()
{
 retrieveListItems();

}); 

I am not able to figure out what is issue. Comment ASAP.