When was the last time you rode? How far? What kind of bike?
The type of bike and the brand of bike don't really matter -- you need a bike you will ride. Does it fit? Do you enjoy riding.
I moved a couple of years ago; but, prior to that, I commuted year-round by bike. (10 miles one way) I rode a hard tail mountain bike -- that was the bike I felt the most comfortable riding in traffic, construction zones, etc. (I had several other bikes, including a recumbent.) Most of the other bike commuters I knew rode road bikes, except for a couple who rode recumbents. It really comes down to what you feel comfortable riding; so, you enjoy riding it, twice a day, every day, in sunshine and in rain.
Consider picking up a used bike first. jppope's Craigslist suggestion is good. Also check your local bike shop, since some have used bikes. Even Goodwill might be an option. See if you really like commuting by bike. If you do, get a nicer back and you have a backup.
Get a reflective vest and wear it. A taillight is better than a rear reflector. Get a good, strong headlight, especially if you are riding at dawn, dusk, or night. You'll use it in the rain, too. Do not get a flashing headlight.
Keep at it and don't get discouraged. It's a great way to commute.