code writing

What i need to know before start write codes for stepper motor ?

You could do worse than starting by looking at the examples that come with the stepper libraries.

Have you got the hardware (stepper driver board) and a suitable power supply (not from the Arduino board itself) sorted out ?

yes i got the hardware
Nema 17
Arduino uno
Cnc shield
Drv 8825
Power supply 12 v dc , 15 Amp
And i need to know or learn how to write code to test the motor .

Some stepper motors don't work properly with high microstepping values. I would suggest initially starting with the driver board set for full step operation. Get it moving and then incrementally increase the microstep amount.

As has been said above. Get started with one of the example stepper sketches.

Some light reading about stepper motors:
Stepper basics.
Simple stepper code.

It is very important to set the coil current limit on the DRV8825 drivers. This Pololu page shows how.