Your little diagram is like a freebody diagram (just an incomplete one). A freebody diagram is a schematic diagram with all of the objects in your system which are subject to or exerting forces with arrows indicating where the forces are applied and with what magnitude.
You need to add to your diagram the weights of the boxes. They should be drawn as arrows coming from the centre of mass (usually just a middle point) of those objects and facing in the correct direction (in this, it will be down towards the ground). Then you can start considering each object and work out what the contact forces should be to preserve equilibrium.
So... consider box L. There are three forces involving L:
1. The weight of the box (8 N downwards)
2. The pushing force on the top of the box (4 N downwards)
3. The contact force of K (? N upwards)
So... if the box is stationary, we know that, because of Newton's first law, the net force must be zero. In other words, the upwards forces must balance the downwards forces.
Can you now calculate the contact force?
After you've calculated that, you can then do the same for box K. Remember that the weight of box L is applied to it and the floor is pushing up on the box as well as the box having its own weight.
To double check your answers, you can do another calculation considering both boxes as a combined system (like one big box with a weight of 12 N) and the forces should still balance.
