While you're still on your own ship (if you get off, just turn around and get back on), Head to the locker room area.

Then turn right and go to the room with all the lockers in it. You're now looking for the one on the right wall that looks like it holds guns. Walk up to it and you will see.



In order to get this without leveling up ages, turn around and look at the corner to the right of the workbench.


You're going to now jump over the crates like Mario and look for a power switch.


Flip that switch and you can now go get your free gun!



Remember to mod it fully and level it up for an achievement!
Maximum Gun: "Fully leveled-up Rookie's Reward. Find the Rookie's Reward weapon early in the game and invest all available upgrades into it until it's maxed out."
Happy slaying.