Thats actually a complex question with a complex answer.
Ancient people were all polytheistic. Abraham was the patron of Judaism. Abraham lived in Ur in Mesopotamia, a polytheistic society, and was likely himself polytheistic. If you have read Genesis, you should be familiar with how God, or yhwh, (who was just another god among their pantheon) came to him and asked him to worship only him, and to leave Ur and go to the land that God had promised him, where god would make him fruitful and make his offspring as numerous as the stars.
At this time, Abraham still probably believed that the other gods among the pantheon existed along side yhwh. As time went by, God began telling Abraham that he was the ONLY God. And all the other ones were fake. This is a constant theme in the Torah. There are numerous instances of the Jews worshiping other gods, and God getting very angry about this. Baal was a popular one that the Jews frequently turned to.
It's actually pretty apparent in the Torah that the early Jews struggled with the idea of there being only one god.
Genesis and Exodus are really pretty easy reads. Not too long, and not too complex like the rest of the bible. If you read them, it will give you a basic understanding of how Judaism came about.