HTML editing

I'm learning how to hack websites, but sadly I've reached an impass. I know what to change in the HTML coding, I just don't know HOW. All I have is the URL. What comes next?

While you are right that roflcat.com needs to be punished (especially for the bitter front-page message where the owner whines about "noobs with sand in their vaginas" complaining), the fact that you have only just learned HTML indicates you will never, NEVER have the skills to actually disrupt another website.