Based on your description of the roads and sidewalks, I'm going to guess you go to school in Boston? If that's the case, space IS limited on most roads, but that still isn't an excuse to ride on the sidewalk.

Like others have implied, I sense that you are somewhat new to biking, not just urban biking. That said, there's some great advice in this thread. Try to ignore the sarcasm and plain-old "mean" comments and you should be off to a good start!