Anyone who bears witness to the oneness of God and the final prophethood of Muhammad (peace be upon him) is a Muslim. In order to be considered part of the community of believers (the ummah), one must publicly testify to this belief. The process for converting is not a sacrament, but simply a public witnessing, in which one states in front of other Muslims that "I bear witness that there is no god but God, and I bear witness that Muhammad is the messenger of God." The public witnessing is then followed by a warm welcome into the community, and the beginning of a journey.
Note: the choice to convert to Islam belongs to the individual person; God says: "There is [or shall be] no compulsion in religion" (Qur'an 2:256).