To communicate with (and send gcode to) the Arduino and its stepper drivers there are several solutions.
You could check the GRBL Wiki. Scroll down and you’ll find plenty of software that deals with GRBL and Arduino.
There are even some nice GUI tools. Nevertheless I used Groovers Gcodesender. Can be found on Groover’s Pocket Laser Instructable (Step 7).
Go ahead and try some g-code commands.
Get your steppers in the correct starting position before powering them (e.g. zero position) and type
G91 G28 X0 Y0
This tells GRBL that the current position is the zero position.
This moves the ‘spindle’ to the absolute position of X50 Y50
G01 X50 Y50
this would move the spindle 50mm on the X and 50mm on the Y-axis from whatever position the spindle currently resides. This is the relative positioning.
For more information on gcode commands, check the wikipedia page (link).