Press the button to the right of F12 (should be there), it'll say prtsc sysrq or something along the lines of that. Open MS paint and paste, then crop it however you want, save it. After that go to in image hosting site like PhotoBucket, Imgur, Imageshack, etc. then upload. Grab the direct link if you want to post it in the forums, press the "image" button above the text box then paste the link.
That's how to do it on a PC at least. I'm not sure how to do it on a Mac.