Hi Aditya, and welcome to the site! There are some tutorials here on this site (Beginner, Intermediate, and Advanced) and there is a wealth of information here in the various articles and forums. YouTube is another good source as it is often helpful to watch a video instead of reading "how to do something". And there are even more Linux training website, some for free or some that cost money. Plus your son can give you good hands-on advice too if he is near enough to visit you.
The Raspberry Pi can itself be a very good introduction to Linux as there are a number of Linux systems that run on the Pi. Try out different ones and let that be your experimental machine for awhile. It is quite capable for such a small device.
There is no "Chapter 1" or "Start Here" place to direct you. Everything you begin seems to jump into the middle for awhile, but it gets better. The best thing is to run a Linux system... on a Pi, or on an old spare computer, or in a virtual machine in Windows, or even just to boot and run on a DVD or USB stick. Once you start to run it, explore it, find out out how to install software and how to update the system... then you have begun the journey. It may be frustrating at times, but its also fun to learn new things.
If you have questions, open a new thread in one of the forums (Getting Started, or General Linux) and we will try to help you along. Good luck!
Cheers