You'll need to supply the distance and wall height for left/right field lines, straightaway left, left-center, straightaway center, right-center and right field. You'll also need month-by-month temperature and precipitation data (easy to get from Wikipedia) as well as altitude (also a Wikipedia thing).
Plug it all in, and it will calculate the park factors as you go. It's interesting to watch them change with each new piece of data you put in.
OOTP Park Generator