Related posts plugin
Note: to setup this plugin you will need to paste the code into the theme editor. The video is explaining that very well. I just want to warn you, whenever you’re changing something in WordPress theme editor make sure to copy whole code into the text editor on your computer before you make any change. After you have the copy of the code on your computer you can safely paste additional code from related post plugin, (what you will see in the video). This way, if something goes wrong, you can always paste back original theme code to your WordPress blog.