I am taking notes of all of this wonderful information. So far I have gleaned the following:

Better to have both a pump and CO2, but if I can only get one right away get the pump.

Inner tube

Light of some kind

Multi-purpose tool (I already have something that should work)

Tire changing kit that will work with my bike (the tires, according to Trek's website are the Bontrager Race Lite Hard-Case Plus, All Weather, 700x28c).

Helmet (of course)

Later on:

Tire patch kit once I start getting out there by myself and have taken a class to learn how to patch a tire.

Is there anything else that a total novice should purchase with her bike? I want to get what I need, but no more than that until spring.