On the right side of your screen, there's this tall grey thing.
It's above a button that looks like V, and below a button that looks like ^.
Now, while your mouse pointer is hovering over the tall grey thing, what you're going to want to do is click on the left mouse button, hold it there, and while you have the left mouse button held down, move your mouse away from you, so the page scrolls up.
Now, notice on my post, you see this big red tomato looking thing with spikes and sharp teeth, when you're scrolling up, you'll see it again, and to the right of it, there'll be these things called words, like the ones you're reading right now. They are instructions, much like the ones I'm giving to you now.
Follow them, and you'll be able to play HDoom, though, to be quite honest, I think you need to go to school.