One of the key issues regarding the cuprate high temperature superconductors is the evolution of the electronic structure when charge carriers are doped into the parent Mott insulator. We have performed scanning tunneling microscopy studies on the parent Ca2CuO2Cl2 Mott insulator and severely underdoped Bi2Sr2-xLaxCuO6 in the antiferromagnetic insulating state. The large energy window covered by the tunneling spectroscopy allows us to simultaneously capture the features of the full charge transfer gap and the low energy electronic state at the atomic scale. We show that with increasing hole doping, the high energy spectral weight of the upper Hubbard band is systematically transferred to the low energy electronic states within the charge transfer gap. When sufficient amount of holes are introduced, a V-shaped energy gap forms near the Fermi level and in the meantime a short-range charge ordering emerges. The implications of these results on the pseudogap phase and charge density order in the cuprates will be discussed.