The magnetic environment of spin qubits plays a key role in their applications in quantum information. For example, while the nuclear spin bath of material acts as a noise source for an electronic spin defect as qubit platforms, the spin-possessing nuclei can also be used as auxiliary quantum memories. They provide an excellent platform for long-term quantum information storage due to their low coupling to the magnetic environment. This thesis aims to investigate the properties of the environmental spin bath in spin defects using first-principles simulations. We first build an efficient computational framework based on the cluster-correlation expansion (CCE) method to model the spin qubit interacting with the spin bath. We then combine theoretical predictions and detailed experimental validations to characterize the noisy environment of spin qubits in materials at the fundamental level. The spin qubits studied here exist in a wide variety of systems - defects in silicon carbide, diamond, oxides, and even in molecular crystals. Over the last fifty years, computing evolved into a third pillar of science, alongside theory and experiment. As such, the first-principles simulations of spin qubits provide a rare opportunity to completely shift the paradigm of how we approach the characterization and interpretation of the experiments on spin qubits from the individual investigation towards the automation and an industrial scale.