I suggest, you check out Nickydude's Community FPSC Guide, It has very nice Tutorials by Plystire on using FPI in the beginning section of the guide. It may be useful to download FPIEditpad as well.
http://www.olivetreegames.com/FPSC/new/FPI_EditPad_3391.zip
On the right side of the program, are a list of all the stock commands, if you hover your mouse over one, it will give you a full description of what that action or condition does. Learn those commands, and writing FPI will be like writing a book
Once you think you have a firm grasp, you can study already made scripts. See what it does in game, and then look at the script and see if you can determine what in the script, makes it do what you just saw