At the risk of sounding snarky, I just want to add one thing to the otherwise excellent advice in the answers: motivation.
Safety considerations aside, you don't really need any accessory to commute in bad weather. It is more a matter of how much discomfort you're willing to tolerate. "The gear" is not what is holding you back. Sure you can go all out and get full fenders with mud-flaps, waterproof panniers, appropriate lights, a cycling specific rain poncho, exactly the right gloves, splats, rain-pants, shoe covers, full-chain-guard, wax lubricant, seat protector, etc, etc. But what would that get you? You'll still get wet or sweaty and you'll have to maintain all this stuff not to mention lug it around-- or risk finding yourself leavinng work during a thunderstorm with all your rain-day-stuff safe and dry at home.
My advice is to get fenders/lights to start with, then just make a commitment to commute in the rain. Eventually, as a result of an extremely uncomfortable commute, you might consider adding a thing or two beyond that and make carefully considered adjustments according to temperature, distance, speed, and portability.
Generally speaking, you're more likely to stick to it if you do it no matter what with the expectation of a modicum of misery and discomfort.