The kernel.org site is down presently, since the servers for The Linux Foundation were hacked recently. Hopefully it will be back up soon. The Linux Documentation project (tldp.org) is up, however, and it has a lot of tutorials, articles, how-tos, and such to help you.
TO start with you can play with some commands that would be helpful for you get a good hands and get familiar with it. If you want to know a OS properly,first be the end user of the system,know the in and out of it. Once you get acquainted with it you can read it from google One preferred website: www.kernel.org
This will help you but getting a hands on is more important .
Install/download the kernel documentation (its mostly plain text files) and take a peek. I did this, for example, to work out how to use Joystick.h in my c++ app. - Thats the kernel gamepad/joystick libary in linux.
It is very confusing if you dont know where to look, I am trying to learn it, but there are too many files. The problem If you like to modify it to serve you well, you need to know what or which file need to be changed, and which not