Explore Linux: Grab a copy of any distribution that appeals to you, install it and explore to your hearts desire.
What languages have been used: For the kernel, almost exclusively C and Assembler (as far as I know). Almost all of the system libraries are written in C, too, with a few in C++. As for the rest it's a healthy mix of almost every language out there.
And I too want to remind you of the Rules, specifically Rule 9, which you agreed to when registering.
What I did was, I got PCLinuxOS as a live cd ISO and burned it. Then I could do anything like edit the code, if you mess up the system, just reboot and your fine.