Ditto what everyone else said. But, just to get you riding, I would recommend the below:

Get a tail light, front light and reflective tape (you never know when you'll get stuck at work late)
Backpack
Basic tool kit to repair a flat or chain
Bike lock (if needed)

All other things are nice to have if you can afford it. If you're looking for the necessities, I wouldn't ride without the above.