Strong light-matter interactions between quantum excitations and confined electromagnetic fields open up new possibilities to impact chemical reactivity and charge transport. When electronic or vibrational excitations are strongly coupled with a photon mode (for example near a plasmonic nanoparticle or in a microcavity), collective excitations lead to intriguing phenomena that are fundamentally different from conventional photoexcitation. Despite recent developments, understanding collective excitation remains challenging from theory and simulation perspectives. In this talk, I will present our recent efforts in modeling superradiance and cavity effects in a disordered system and our future endeavors of developing a theoretical toolbox for simulating collective excitation in materials.