I am new to NopCommerce v2.4 and wondering where do I write my code (by creating new model in admin or nop.web section)

http://www.nopcommerce.com/docs/75/plugin-with-data-access.aspx

Thanks for the link but I still need some more help.

I added that project in my Plugins and do I have to do anything else?
I am not able to see this plugin in admin -> Configuration -> plugins

I added following line in \Presentation\Nop.Web\Views\Catalog\ProductTemplate.SingleVariant.cshtml
            @Html.Action("Index", "Tracking", new { productId = Model.Id })


I am getting this exception:
Execution of the child request failed. Please examine the InnerException for more information.
InnerException: {"The controller for path '/p/5/1-oz-krugerrand' was not found or does not implement IController."}

I just added few products through admin.

I will appreciate if you can guide me.

Added by mistake - not sure how to delete this post.