Charge density waves (CDWs), the periodic modulation of charge density arising from electron-electron interaction and/or electron-phonon coupling, have been central themes of modern condensed matter physics, as studied in multiple compounds including transition metal dichalcogenides, cuprates, nickelates, and recently kagome metals. Among them, CDWs in kagome metals have attracted significant attention due to their intriguing characteristics including time-reversal symmetry breaking and their interplay with superconductivity and nematic phases. In this talk, we present our comprehensive first principles study on kagome metals ScV₆Sn₆ and LaRu₃Si₂ that host multiple competing CDW instabilities. We carefully identify competing CDW states by considering multi-phonon effects. We discuss the crucial role of finite temperature effects through anharmonic lattice vibrations in the formation of CDWs. We also show various phase diagrams highlighting the role of lattice parameter, strain, and pressure. Our theory is consistent with experimental reports.