First-principles density functional theory calculations are performed to understand the electronic structure and interaction parameters for recently discovered superconducting kagome metal CsV3Sb5. A systematic analysis of the tight-binding parameters based on the maximally localized Wannier function method demonstrates that the out-of-plane Sbout-p orbital is a key element in complete description of the three Van Hove singularity structures known in this material at the M point near the Fermi level. Further, the correlation strengths are also largely determined by Sbout-p states. Based on the constrained random phase approximation, we find that the on-site and intersite interaction parameter are both significantly affected by the screening effect of Sbout-p orbitals. As the role of this previously unnoticed orbital state can be tuned or controlled by out-of-plane lattice parameters, we examine the electronic structure and particularly the evolution of Van Hove singularity points as a function of strain and pressure, which can serve as useful knobs to control the material properties.