This chapter focuses on the practical application of DFT in molecular systems. We discuss the process of “geometry optimization” and the idea behind it, which is the very first step of every DFT calculation. We introduce the Gaussian, a popular software program to perform such calculations. We continue to discuss the capability of this program with a brief theoretical background, wherever needed. We talk about several kinds of calculations to be performed by the Gaussian such as thermodynamics, population analysis, NMR, NLO, NBO, TDDFT calculations to name a few. More importantly, we discuss how to perform these calculations, extract, and interpret the results. Ideally, this chapter provides all the ingredients needed to grasp the results discussed in the forthcoming chapters.