If you're a PC user and you have a full keyboard then you have it lucky as you can generate a screenshot with the press of a key. This key is usually labelled as "print screen" or "prt scrn" for short. See example below:

However, if you're on a PC then once you click this key all you're doing is essentially copying the screen grab onto the clipboard, so you will have to paste it into an editing program in order to save it as a picture.

I recommend using MS (Microsoft) Paint as it's generally on most PC's and if not it's a free software, so you can always download it.

Now once you enter MS Paint all you have to do is select the selection tool, create a selection and then hit paste, and voila! you have your screenshot. Save this image as a .jpeg and you're ready to upload!