The easiest way to get a screenshot is: push the button "print" on your keyboard.
Then open a paint editor, open a new file and paste the screenshot in that pic.
Look for the paste button or just press "Ctrl" and "V" on your keyboard.
If you can save it as png or jpg or gif it is good, because those files are quite small.
Don't know whether you know how to upload a pic in the net, though.