During character creation, you’ll be prompted to choose your Pre-Service History, or colloquially, your origin. The only major gameplay difference between the three origins is what origin-related assignment you’ll get, which itself is a fairly minor quest. You have three origins to choose from:
Both of your parents were in the Alliance military. Your childhood was spent on ships and stations as they transferred from posting to posting, never staying in one location for more than a few years. Following in your parents’ footsteps, you enlisted at the age of eighteen.
A somewhat unique origin in that it’s the only origin where Shepard will have any known family. You’ll have some minor correspondence with your mother in the sequels, but otherwise the only effect this origin has is it’ll lead to the assignment [Citadel: Old, Unhappy, Far-Off Things].
You were born and raised on Mindoir, a small border colony in the Attican Traverse. When you were sixteen slavers raided Mindoir, slaughtering your family and friends. You were saved by a passing Alliance patrol, and you enlisted with the military a few years later.
A traumatic backstory that pains Shepard as a tragic survivor with a good incentive to join the Alliance military. By choosing this pre-service history you’ll get the assignment [I Remember Me](I Remember Me “I Remember Me”).
You were an orphan raised on the streets of the great megatropolises covering Earth. You escaped the life of petty crime and underworld gangs by enlisting with the Alliance military when you turned eighteen.
A rags-to-respectability story, which is a fine excuse to explain an unorthodox Shepard’s questionable approach should you opt to go down the Renegade path. But this is all head-canon and roleplaying. Gameplay wise, it’ll give you access to the assignment Citadel: Old Friends.