If you find that your Z axis endstop is triggering with the nozzle too high from the bed for a good first layer squish, there are several methods of setting the offset in either software or gcode. 

One such option is to insert a G92 Zxxx command in your start.gcode. This sets the Z axis xxx units above the bed and as such when the printer starts printing, after it has homed all axis, it will move down to 'zero' to start printing. 

Start by homing all axis with the G28 or 'home all' command and then measure the nozzle offset from the bed. If you find this to be, for example, 0.7mm then you would add the following line at the end of your start.gcode: 

G92 Z0.7 ; 0.7mm Nozzle Offset from Bed

Your next print should now print correctly :) If not, simply keep adjusting the offset until you find the perfect first layer height.