If you’ve never played a Mass Effect game before, don’t worry about it. You don’t need to know a shepherd from a Shepard or have an in-depth knowledge of the original trilogy to dive into Andromeda.
The choices made at the end of Mass Effect 3 have no bearing whatsoever on this new narrative. That’s because Andromeda’s characters departed the Milky Way in 2185 (the same year that marked the start of Mass Effect 2) meaning it’s entirely possible that the entire fleet has zero knowledge of what happened to the original protagonist, Commander Shepard, or the fate of their home.
Hence Andromeda is essentially a soft reboot, taking place 636 years after the events of the original in a totally different galaxy: Andromeda, rather than the Milky Way.