It's been said, but FENDERS! And they're not worth cheaping out on, IMO, if you get rain much at all. Get both front and rear if you're going to do it, because the front wheel will cover your shoes and lower legs with grit and spray otherwise. I'm a fan of tightly wrapped fenders that cover a large chunk of the wheel, but I'm also commuting in Seattle, and the rumors about this place are false - it rains a lot more than people say. Or, at least, is wet in the mornings.
For lighting, it serves two functions: See and be seen. The second is a lot easier than the first. A flashing LED light front and a rear red flashing LED light cover "be seen," but are rubbish for seeing at night (depending on where you ride).
The good news for lighting is that you can find a lot of good stuff on eBay for very little. USB rechargeable lights are nice, but a bit pricey, and I'd rather have 2-3 cheap lights than a single really bright one (for being seen). Don't spend a lot of money on lighting until you're sure you're going to ride a lot, though.
If you're planning to wear a helmet, I'd also invest in a neon helmet cover (or just buy a neon helmet). I have this one, and am quite happy with it:
http://www.ebay.com/itm/Neon-Green-Helmet-Cover-/251745158152?pt=LH_DefaultDomain_0&hash=item3a9d2e4408 It makes a big difference in people seeing you from a distance, which, at least for my commute, is a big deal.