Congrats. As a recent (6 months, steady) commuter, I would echo much of what is said above. I started out riding 1.5 miles each way, in street clothes, with no cycling extras (tubes, pumps, etc..)
I began wearing cycling oriented clothing when I decided to ride further - originally I did a longer ride on the way home, so I added cycling shorts under my casual work pants. Then I decided to ride further one or two days a week, followed by the need to ride further every day (6 miles each way), which quickly convinced me to carry my work clothes, and wear wicking shirts with my shorts (I wear baggies over spandex -- don't want to look to 'racer boy'). I don't have a shower option currently, but I shower before hand, and try to ensure adequate 'cool down' time before I need to get down to business. I work in a casual environment, with very limited client contact, so I don't need to 'look my best' very often. And I have the option to drive if necessary.
My second flat convinced me that spare tubes, a patch kit and a pump were near necessity, unless you have a guaranteed pick up. A broken chain several weeks ago convinced me to buy a multi-tool and carry a spare master link (but I haven't actually done it yet).
My key issues right now are 1)Visibility: early darkness has convinced me that a new light set is in my future, and I recently bought some reflective vests ($6/ea, including shipping, from Costco.com) and 2) carrying capacity: I think I'm going to join the rack/panniers crowd, and lighten the load/eliminate in my backpack, as I now ride all the way to my office (11 miles each way), which reduces my need to carry stuff off the bike.
Anyway, stick with it, and you'll learn what works for you. One thing I've definitely learned hanging around here is that each of us has a different set of requirements and tolerances, but you'll find advice for all of your needs :-)