Genesis begins with the early years of creation. It tells about God creating heaven, earth, and the first humans. It also tells about the first time humans sinned. This caused humans to be separated from God and to eventually die. Genesis 1-11 briefly tells about other important events that occurred over many hundreds of years.
Genesis is also about the beginning of God's people. Genesis 12-50 tells about how God remained faithful to Abraham and his descendants. Abraham's descendants became known as the Hebrews and later as the Israelites. These people would worship Yahweh and be his people.
The writers of both the Old and New Testaments presented Moses as being very involved with writing the book of Genesis. Since ancient times, both Jews and Christians have thought that Moses wrote Genesis, Exodus, Leviticus, Numbers, and Deuteronomy.