In Genesis, Shem is listed first among Noah's three sons. Genesis 5:32 names Shem, Ham, and Japheth in that order, and Genesis 6:10 repeats the same sequence when describing Noah's family.
The order is a textual fact, not a statement that Shem was necessarily the oldest. Genesis 10:21 uses wording that has generated translation and interpretation debates about Shem's age relative to his brothers. Consequently, readers should distinguish the first name in the list from a definitive claim about birth order.
Shem becomes especially important in the genealogies that follow the Flood. Genesis 10 associates peoples with Noah's sons, and Genesis 11 traces the line from Shem toward Terah and Abram. Those later genealogical traditions are why Shem's name appears frequently in discussions of ancestry, but this question concerns only his first position in Genesis's list.