Welcome to bike commuting! It's the best part of my day, and I hope it'll be the same for you. You don't need the following things to start, but I have found them handy over the past 10 months, adding in gradual stages as the weather changed.

If you are going to commute in the rain, I strongly recommend fenders--they really do cut down on the amount of water you get. You can wait and see how you feel about riding in the rain. I rode in everything this past winter and found that made riding in mild rain a lot harder to face, through precipitation fatigue.

A rear rack is handy, and can act as an intermediate fender if you don't want to go "all the way" right now. The wet & muddy stripe up your backside isn't too pleasant...

Riding at night? Lights. Lotsa lights. headlight, rear blinky, blinky for your bag, I even clip one to my jacket. Reflective tires are wicked cool and really are SEEN. I love them, but I don't have them.

I like the Jannd reflective trouser straps; they are very handy tying stuff down when I don't need trouser straps. But I got tired of catching the pants in the bike parts and these do help. I don't think they are great all day wear, from what my co-workers tell me

Lastly, panniers. Moving weight off my back was a big help. When they have reflective strips, all the better! there are lots out there to choose from at all price ranges. If you leave the bike out in the rain, make sure that the zipper flap is covered!