Hello...to begin with, I think it will be good to browse through the manual and at the same time take photos to learn the different functions.
When I bought mine I did this for few days to get accustomed.
However, having said that, the more photos you will take with your flash the more you will know the way it behaves. Honestly I believe I am yet to know the SB900 properly. I have it for almost 5 months.