The extraordinary interest in two-dimensional (2D) materials is increasing day by day. Thanks to advances in the experimental techniques, monolayer form of another material is synthesized every day with features not seen in the bulk form. Ab initio methods provide useful tools for characterizing and functionalizing the various properties of these materials. The results obtained through first principles quantum-mechanical calculations can help to predict and understand the experimental data, such as the position and source of the spectroscopic peaks in the Raman or optical absorption spectra. The aim of this thesis is to predict and functionalize the optical and electronic properties of atomically thin layered materials using density functional theory and approaches beyond. Within the scope of this thesis, possible technological applications of various 2D materials ranging from perovskite crystals to transition metal dichalcogenites are investigated by using several functionalization methods. In order to accurately predict the optical properties of these materials, it is very important to use approaches that take into account the many-body effects. Recent studies have shown that many-body perturbation theory in the form of GW approximation is highly reliable to calculate the quasiparticle properties of materials. By solving the Bethe Salpeter equation on top of GW calculation, the quasiparticle energies and excitonic properties, which have dominant effect in the optical properties of ultra-thin materials are examined in detail.