you press the print screen button on your keyboard, go to a program like Paint or Photoshop, and paste it (there should be a paste option in the toolbar, if not hold ctrl and press V, and that should work), then crop it if you need to and save it as a gif or jpg.
Then upload it to like photobucket.com or imageshack.us and they'll give you an address for it and the coding. You'll need the coding that says (img) only with [ instead of (. Copy that code and paste it here.
I hope that helps. I'm not very good at explaining things.