M92 to set your extruder steps per mm http://reprap.org/wiki/G-code#M92:_S...steps_per_unit. Calibrate as per the reprap wiki guide [url]http://reprap.org/wiki/Calibration#Extrusion[url].

Your initial setting should be based on the type of extruder you have. I have a Wades, set to 711.11 steps per mm for the extruder. A direct drive extruder will have a smaller number.