Develop a wordpress plugin for admin panel

Hello

I am learning how to write a plugin for admin panel for wordpress. I am referring to https://www.devlounge.net/code/constructing-an-wordpress-plugin-admin-panel this link.

However its not working for me.

Can someone share links or tutorials with me which tells how to write plugins for admin panel for wordpress 3.

I want to have a widget on admin panel which allow to filter posts to be displayed by selecting tags and categories.

Thanks !!