Alien ships come to Earth in the middle of the night and kidnap people who must prove themselves worthy of being captain by killing the last captain. The hero is a teacher I believe. The ship he becomes captain on actually took and killed his kids first. It’s lots of ships all over earth finding earthling captains so they can defend the earth against an alien fleet
Swarm by B.V. Larson
Earth arms marines with alien technology and builds its first battle fleet!
Kyle Riggs is snatched by an alien spacecraft sometime after midnight. The ship is testing everyone it catches and murdering the weak. The good news is that Kyle keeps passing tests and staying alive. The bad news is the aliens who sent this ship are the nicest ones out there....
SWARM is the story of Earth’s annexation by an alien empire. Long considered a primitive people on a backwater planet, humanity finds itself in the middle of a war, and faced with extinction.