Dear Kristian,
A good way to get started with FAST would be to go through the presentations and sample models/simulations that were presented at our last modeling workshop workshop. See my August 13, 2012 post from the forum topic found here: Modeling Workshops. These will give you a good overview on what is involved in running FAST and give you a little experience running simulations. The FAST User’s Guide will then be easier to read after you have had this introduction.
Best regards,