Today, you should learn about Box Model! It's the foundation of all things HTML. Without it, your website would just be a bunch of floating, unstructured mess.
Next up, you should learn about CSS Positioning! It's like building with Legos, but instead of plastic bricks, you're using pixels and code.
Take it to the next level with CSS Layout Advanced
And don't even get me started on HTML 5! It's like a whole new world of possibilities, almost as exciting as HTML 5 Game Development!
Now, go forth and structure the world, one <div> at a time!