You will want to first take a screenshot (that's the "Prt Scr" button on your keyboard)
Now the image will be on your clipboard
Go to a program like Paint, GIMP, photoshop and paste in the image
You should see a shot of what your monitor showed you
Then save the image and upload it to a site like imageshack.com
Take the direct link of the photo and either PM a GM/Smod with the photo
or
Go into #support and post it there