Get a good helmet. If you're riding in the dark, put a red flasher on your rear, a white on the handlebars, and a headlamp on your helmet. The headlamp is the best because it will flash a light where ever you look. So if someone is pulling out of a side street and your worried they don't see you, you can just look at them and know you're shining a little light their way.
You will want to be able to carry things back and forth. Either get a good quality bag made for cycling, or mount baskets or panniers to your bike. Your back will thank you. If you plan to ride in the rain, pick a waterproof bag so the contents stay dry. Also invest in a nice jacket and some water proof pants. Bike in shoes that will get wet, and carry dry shoes for work in your bag. If you bike when it is wet out, either wear your full gear (rain jacket and rain pants), plan to change when you get to work, or get full front and rear fenders for your bike. Depending on the bike, these may not be available. Mountian bike fenders will protect your top, but not your bottom.
If you're a coffee/tea drinker, look for a handlebar mounted bottle holder. They work great for holding your morning pick me up. Or get a thermos and carry it in your bag.
Invest in a good quality D lock as well as several cable locks. Leave them locked to the bike rack at work. Use the D to lock the frame of your bike to the rack, and the cables to lock both wheels, the seat, and your helmet (if you want) to the rack.
Have a plan B in case of emergancy. If you need to get somewhere fast, do you have money for a taxi? A friend that could give you lift on their break? While it is unlikely to happen, it is good to make sure you're prepared just in case. Carry a cell phone and have a backup incase of some sort of serious problem, like two flat tires. Know who you can call to come "rescue" you if you really need it. (This is just like driving an old, unrelaible car.)
Enjoy the commute! Biking to and from work was the best part of my day when I did it. I always arrived at my destination mentally and physically ready for what was ahead.