1. build routines for LCD drivin. Somthing like - x,y(where to display), number(what to display), format(how must be formated - floating integer and so on)
2. Build routines for keypad reading
3. write math library for all functions you need.
4. make state machine for desition, when andwhich function to call and how to display result.

Thats all. but realy realy a lot of work with math. everything else is more than simple :)

Have fun!