At tldp, I would start with the Bash Guide for Beginners, then "ABS" later.
easier formular(if there is)for learning shell scripting.
Easier than what?? We have no point of reference as to what you know already, and how you are trying to learn.
I'll tell you what I think is the ONLY formula: Open a terminal and start typing commands. If you don't know what command to use, type "man -k <keyword>" After playing around for a while, then sit down with a book (such as the one above)