Screen shots are taken just by taping the button that has "Prt Scrn" on it when you are on on the screen you want to copy. You then need to go into a graphics program (paint or any other will do) and paste it as a new image. Then save that new image. That is the only way to be able to then share it.
To post online you would need to upload the image to a site that will allow for linking to your thread. There are tons of sites for hosting images.