This case study focuses on the process of acquiring the language of a four year old Indonesian child. This study also analyzes several other factors and mechanisms that influence the acquisition of a first language. For nearly five months, data gained in this study by observing and recording subjects in producing sounds, words, and sentences during in the middle to the end of the 2019 year. We also see how the child interacts with various kinds of input sentences to see how he processes the language. In the end, we also observed him when he was interacting with other people to determine the level of constructing various kinds of language communication competencies. The analysis of the data was anchored on the confirmation or rejection of language acquisition theories, principles, and previous researches. Based on the findings in this study, the child who was the subject of this study obtained Indonesian as the first natural way. Evidence obtained from observations states that the child uses the Language Acquisition Device (LAD) well, which is the main thing that helps him acquire the first language. Also, the environment plays a vital role in activating innate capacity in language acquisition. Observations and interviews were carried out with her parents to ensure that she acquired the language by imitating and learning through corrections and reinforcement assistance, analogies, and grammar input from the parents. The child also proved that their environment greatly influenced difficulties in language and communication. Natural processes and environmental influences also play an essential role in acquiring the first language in children. Parents and teachers should consider this process by creating good language learning and development at all levels of learning.