Well, a good place to start would be to analyze the traffic between your browser and YT while watching a video. Then learn all you can about that traffic and what it means. This will require a lot of time, testing and sleepless nights. Then you'll need to learn and have several programming languages in your repertoire. If you're intelligent, I would suggest PHP, Javascript and C++ for speed and efficiency. Same holds true for hacking any other site. But if you don't know this basic knowledge to begin with, you've got a long road to travel.