If you use a normal pc, try this, press Print Screen. This will take a screenshot, but not save it. To save it, open Paint, click on edit and then select paste. Your screenshot will then load and you can save it.

If you want to edit the shot, Paint is not very good. I use Irfanview. It is a good editing program and is a free download. (sorry no links as it is against the rules)

If you want to post an image on this forum perhaps, you will need an image hosting site. I use photobucket as it is easy to use and free. There are others but some only post thumbnails.
Once your image is uploaded onto the image hosting site just copy the IMG code and paste it in where you want the image, like this:



With a bit of practice it's easy.