Mikael,
It very much depends on how your computer is set up I'm afraid. However, the general idea is that you get your computer 'desktop' displayed across the two screens (and you should be able to find instructions for this bit in your Windows Help section or by searching on the Internet - try putting "how to set up dual monitors" into Google and I'm sure you'll find articles to help you).
Once you've done this, decide which bits of your FS cockpit you want displayed on the second screen (the GPS, Comms or ATC window, external programs such as IVAP or FSNav etc etc) and then open these windows within the main window. Right click in this part of the cockpit and a menu comes up - click the 'undock' option and then drag the window to your second screen area and resize it as necessary.
Hey presto! You have whatever windows you want on your second monitor!
If this doesn't answer your question, please say and if you can include a bit more information about your computer specifications and exactly what you want to do, it will help us to help you
