Question: First I need to programmatically get tags (unlike what is done here on stackoverflow) from the titles and descriptions of my posts. I don't want commonly used words to appear in keywords. Is there an efficient way of doing this? 

After getting good tags, I would like to save them in MySQL DB.

Now, how do I **efficiently** get related posts using these automatically created tags? E.g. What is done here.

Answer: 
Look up [tf-idf](http://en.wikipedia.org/wiki/Tf%E2%80%93idf). You're looking for terms with a high tf-idf score.
I would advise against using this method. You can use it to *suggest* tags, but automatic tagging will be very, very hard to implement correctly and accurately. 

One of the reasons for that is because computers don't understand semantics. Take any question here and try doing that. It will not work 95% of the time.