As I mentioned in the Software Testing Best practices, Providing Screenshot of error message and screenshots of each step for reproducing bug will help to fix the bug easily.
PrintScreen Key will be useful for taking screenshots. You can simply press this button to capture the image of screen and you can paste it in your word document or paint program.
But it will take the entire image of your screen. i.e It will include screenshots of all windows in your computer screen.
It will be nice if you take screenshot of only the Active window.
It can be easily done by pressing Alt key along with the print screen key.