Buddhism is a missionary religion, meaning like Christianity and Islam, one “converts” to Buddhism. All you have to do is recite the Three Refuges, known as the Three Jewels:
I take refuge in the Buddha.
I take refuge in the Dharma.
I take refuge in the Sangha.
Sangha means community. Dharma means “duty” in Hinduism and refers mostly to teaching, though it can be translated as well to “the way it is”; hence touching on the notion of expressing truth or reality. Finally, Buddha is the human being who, through merit and practice, achieved nirvana. He is thought to be divine, or simply a teacher, depending on different strains of Buddhism, and his namesake “Buddhism” is a derivative of his teachings and insights.
So, the Three Refuges collectively speak to community, duty, and the path that the Buddha laid out.
Two other core aspects of Buddhism are the Four Noble Truths and the Eightfold Path — check out my concise articles on those subjects here!