If your Home switches (can be limits configured also as home) are at the normal positions, ie table fully right, fully towards column and Z fully up then set the SoftLimits as follows.
 X and Y min and Z max set to zero
 X and Y max  set to the travel of each axis
 Z Min set to travel of Axis but a negative number.
Enable Softlimits on main page and you should be set.
You will also need to set up a slow zone for each axis, this is machine dependant so you will have to experiment by jogging towards a limit at rapid speed then seeing if the softlimits can decelerate in time before a limit is hit, just keep messing until you find the minimum distance that will keep you from hitting the limits.
Hood