If an error message or something else unexpected appears on the screen and you want to ask someone for help, document this error by taking a "screenshot" and saving it. To do this, press the "Print" key (Windows) or Cmd+Shift+4 and then the space bar (Mac OSX). This will save the current screen content to the clipboard (Windows) or place it on the desktop (Mac OSX). Now open an application that can process images (e.g. Word, Paint) and paste the "screenshot" in a suitable place by selecting "Paste" under the "Edit" menu item in the application. The screen content will then be inserted into the document and you can save or send it for documentation.
Information about which actions were previously performed is also very helpful.