Everyone has already given you some great advice but I have another suggestion for you.

A good way to get initially aquainted with the gears and what they do is to feel them at work. Take your bike to a quiet area (probably not easy to find in Boston!) like a parking lot early on a Sunday. You want to be where there aren't any big hills and where there is no traffic to worry about. Then, start riding. Ride in a fairly strait line and try shifting one lever at a time. You'll be able to feel how it gets easier and harder to pedal depending on which lever you use. Basically, the levers on the right hand change the rear gears and one ones on the left change the front ones. One lever on each side makes it harder to pedal and one makes it easier. The back gears have smaller increments of change, so you want to play with those first (the right hand).

Once you've gotten a feel for which lever makes it harder or easier to pedal, then you can start to imagine how they'll work going up or down hills or on flat road.

Another idea is to flip the bike over so it's resting on the handle bars and saddle (do this on a blanket, grass or carpet so that you don't scratch anything) and as you move the pedals, shift the gear levers and watch what happens. This won't help you understand which hand does what because everything is upside down, but it will help you get an idea of HOW they work.

To learn the finer points of when to shift and how to make things work the most effeciently, the best thing to do would be either take a class, or start asking questions of local cyclists!

Best of luck...and HAVE FUN!!