Did you try to google "related post plugin wordpress"? Well i used "YARP" related plugin, it was awesome.
try to google word "wordpress Related posts Plugin" you can find many plugins, just download and try it in local 1st...
I use YARPP. I find it is really easy to customize using the template feature and is quite accurate on finding related posts, provided you have enough content! Also, as there are many people using it, there are quite a few resources and forum threads. This site I am developing uses a customized YARPP template to show related posts. (See grey box at bottom of article title You Might Also Like). http://darwinhub.com/darwin-airport-resort/