I have decided to write a program in BASIC for the Nspire that will calculate damage in pokemon games. I want it to be able to be run as a library from the scratched. How should i start on this project? 

EDIT: Here is an example of a damage calculator. http://pokemon.marriland.com/damage.php
I want this on my Nspire.

Here is a link with all the minute details of damage calculation. http://www.smogon.com/dp/articles/damage_formula
You probably won't want to make it this complicated but it should have all the formulas you need. (This is for Diamond and Perl so the formulas could be slightly different depending on which generation you're writing the program for for)

It depends on how you want it to be used. If you want the inputs passed as arguments, I would say make a library full of functions. If you want the user to enter information in text boxes and such I would make one large program. Of course you could make both using your library functions in a program that collects the input from the user. Then you could use it both ways.

Good Luck!