Zoroastrianism (based on the teacher Zoroaster) which emerged in Persia claims to be the oldest monotheistic religion, perhaps going back as far as 2000BCE. However, it did not enter history until the 6th century BCE. It was the leading religions of the time and the fact that many features such as messianism, judgment after death, angels, heaven, and hell, free will and the devil were passed on to other monotheistic religions shows its anciency. Elements of its religion can also be found in Buddhism and Greek Philosophy.
There was also an early attempt in Egypt to move towards monotheism. This happened during the reign of Akhenaten (1353-1336 BC)., but after he died the faros who followed him reverted back to their old gods.
The Jews also started of as a pagan society but during the 6th century BC Yahweh emerged as the main god and soon, he became the only one.
The real take-ff of Monotheism happened with the arrival of the Jewish teacher Jesus and later with Muslim teacher Mohammed. In the East, another great teacher was Buddha, however he was teaching more a noble lifestyle than that a religion.
There were many more of these teachers but these three had a lasting effect on future developments. For example, with the collapse of the Persian Empire the Zoroastrian religion, lost its influence but there are still a few pockets of this religion that survive.
As mentioned, this was a period of significant change from individual tribes to more organised societies. These new societies had different requirements, and these teachers had strong humanistic and spiritual believes that fitted these new societies.
From a historic point of view very little is known about those ancient (religious) teachers. Most of the information that we have is based on stories that were told and recorded later, often even much later. With the intellectual limitations of these times it was rather easy for the rulers of the day to create their own stories around them and to use the teachings of the teachers for their own purposes mainly to control people in these new and emerging societies. Some used the teachings in a way that were supportive for society, others twisted them for their own good. There are plenty of examples of both, in the Roman Empire (since Constantine), Frankish Empire (Charlemagne), Persian Empire, Muslim Empire (Caliphates). The new teachings based on one centralised religious concept fitted these new societal and their rulers much better than the old far more liberal and diverse pagan stories.