This is a tricky question because, while Avrohom and Sarah were definitely the first Jews, there are some senses in which the Jewish people did not formally construct itself until later, with the giving of the Torah. So, in response to the question of "what religion people converted from," there are two different points at which they could have "converted."
1) Avraham and Sarah converted to Jewish-style monotheism from the ancient Semitic polytheistic religion of Avraham's father. Although there had been monotheists before (such as Adam, Eve, and Noah), idol worship was de rigueur in the land during Avraham's time.
Avraham had personal revelations which drew him toward belief in a Single Gd--causing him to rebel against his father--and eventually into covenant with that G-d. He then became the patriarch of a family and leader of a large group of followers, all monotheistic.
Not all of his followers nor of his descendants are considered "Jewish" by us today; we retroactively consider "Jewish" those who planted seeds for what has become our modern tradition.
2) When Divine Revelation was given to Moses at Mount Sinai in 2448 (1313 B.C.E.), the Jewish people came to exist in its current sense as a formal, proactively-defined religious and ethnic group. The basic laws, rituals, principles, and organization of Judaism as they are understood today were communicated by G-d to Moses, and thence to the Jewish people, at that time. This body of knowledge is what we call the Torah; it is how "Judaism" has been defined ever since.
So, as far as "what [the people] were converting from"--at that time they were converting from a general tradition of holiness and righteousness within monotheism, begun by Avrohom and Sarah, to a formal and codified system for the service of G-d, begun by Moses. This formal and codified system includes laws and rites that are considered central to Judaism today, including Shabbat, kosher diet, prayer, and the rules for charity and treatment of others.
To complicate all this: Jews have a traditional belief that Abraham and Sarah (along with some people in earlier generations, including Adam and Eve) followed the laws of the Torah in their entirety from the very beginning. That is, Avraham, Sarah, etc., already lived according to the laws that were told to Moses and the Jewish people only many years later. This fairly subtle and mystically-oriented belief, and how it squares with the idea that Judaism formally conceived of itself in 2448, are explained here.