You're a newb and you had gumption to go for a single speed? Hats off to you! Honestly I wish I would have picked a single speed for my first bike.
For a bike like that the first thing I would throw on it is a set of Ergon grips with bar ends. Ever since high school I've never ridden a flat bar bike without bar ends, they're an absolute must for climbing! But don't position them straight up like I see many people do, how can they use them like that?
A blinky tail light is a must (Planet Bike Superflash), perhaps a blinky head light as well. Bicycle computers are cool (I always use the Planet Bike Protege' 9.0). Also fenders are a must if you plan on riding in the rain like we do up here in Seattle.
Awesome bike you purchased, have fun!