Do you have a min z-endstop? If so, you should adjust its location so that the hot end (at its z-home position) is a bit less than the thickness of a piece of paper away from the bed. I.e. if you slide a piece of (thickish) paper between the hot end and the printing bed/glass, you should feel some friction.
Levelling the bed is slightly different in that (after you have done the above at one point on the x-y plane) you adjust the springed bolts of the bed so as to feel the same friction on the paper as you move around in the x and y directions, keeping z at z-home.
In practice, the usual way to do this, is to go at home (i.e. home all axis) to adjust the position of the z-endstop. Then move in steps of say 20mm in the x direction until you reach the opposite corner, adjusting the corresponding bolt so as to keep the friction the same. Then move in the y direction to reach the third corner in the same way. Then move in x to get to the 4rth corner. You'd better go around twice, making further corrections, because a plane is defined by 3 points, so if you have 4 bolts to adjust then every time you adjust one you also affect the others.
