It looks like a nice bike. Enjoy it by riding it a lot!

A U-lock and cable is recommended as an affordable locking strategy. Loop the U-lock through the frame and rear wheel, and then the cable through the front wheel. You should be able to pick up the combo for about $35 or a little more.

The most recommended accessory for a commuter bike is a rear rack. You can strap a backpack directly to it, use a milk crate, or spring for some panniers or trunk bag. A rear rack gives you a lot of versatility. I think Topeak racks give a good 'bang for the buck'.

Another thing people like on commuter bikes is fenders. They are definitely nice to have on wet roads or trails.

Thinner road tires are more efficient on pavement, no doubt about that. Depending on the length of your commute and the terrain, it's something you might want to look into for the future. For long rides every little bit of energy saved helps. If your commute is on the short side, in that case you can probably just wait for those tires to wear out before upgrading.

The more you ride your bike, the more you'll enjoy it. So ride often!

Edit: Almost forgot. If you ride at night you are also going to want some lights. Riding at night requires active lighting in addition to reflectors.