First-principles calculation was used to study the structural and electronic features of the low-dimensional oxide structure, SrTiO3/Sr1-xLaxTiO3 (x = 0.25) superlattices, constructed by submonolayer low-dimensional La doping into SrTiO3. We demonstrate a dimensionality crossover from three-dimensional (3D) to two-dimensional (2D) electronic behavior in the system. Two types of carriers, one confined to 2D and the other extended, are induced in the low-dimensional structure, leading to distinct tunable (3D --> 2D) transport characteristics that will enable the study of many properties (e.g., superconductivity) through this change in dimensionality.