There is a button near the top of your keyboard that says something like, "Print Screen / SysRq". It's typically near the F12 key or above the number pad.
Simply click that button and then open something like paint, right click, and select "paste". Then save the picture, under whatever name, it doesn't matter.
Then open up imageshack.us, and upload the photo.
Copy paste the forums hotlink into a reply here and you're done.