To the right of F12 there's a button marked "print screen" or some similar abbreviation. You push it and the image is copied onto the clipboard, and can be opened/pasted with any image program.
I use Irfanview and XnView, a couple of freebie image programs, for most all of my dabbling, but you can do it with Windows Paint.
http://www.irfanview.com/
http://www.xnview.com/
To post it, there's several free sites. I used this one, a no frills place:
http://www.uploadyourimages.com/
Thanks a bunch!