In this work, in the context of coupled-mode theory, I studied the discrete diffraction and the self-focusing effect in nonlinear optical lattices, under single-cite excitation. In the one and two-dimensional discrete diffraction, the spreading of the beam occurs due to the coupling between adjacent waveguides, while for a critical power and above the effect of self-focusing occurs. I studied the output distribution intensity for periodic, diatomic, and random lattices in one and two dimensions. It is observed that the amplitude, in which the self-focusing occurs, for periodic and diatomic lattices, has the same value. For random lattices with weak disorder, the amplitude is again the same, while for strong disorder, the beam does not diffract at all.