Press these two buttons at the same time. The second button mentioned on this image is usually above the 6 on your keyboard.
You'll see a screen pop up in the bottom right corner telling you the image is saved and that it is in your clipboard.
You can usually paste the image from your clipboard simply by clicking into the area you would normally type in and pressing ctrl + V. Or you can click to add an attachment, navigate to the area your screenshot saved in, and attach it. Sometimes this icon looks like a paperclip.
