7) One last thing
A bicycle is considered a vehicle, and as such, they are required to follow the same traffic laws that cars do. Ride on the right side of the street. Stay off of the sidewalk (unless expressly allowed in a particular location). Stop at red lights. Stop at stop signs***. Ride predictably and signal your turns when warranted. The majority of bike/car collisions are partially or entirely the cyclists' fault. Two of the most common causes of crashes are bike riders riding on the sidewalk, and bike riders riding the wrong way (on the left side of the road). Another common cause is lack of visibility on the part of the cyclist. Eliminating these few (totally controllable) factors actually makes riding a bike statistically safer than driving a car. The thing most new cyclists worry about - getting clipped from the rear by passing cars - is actually relatively rare. Crashes happen primarily at driveways and intersections, and they happen because the cyclist was somewhere the driver didn't expect them to be.
If you are in the SF Bay Area, consider taking the FREE traffic safety course sponsored by the local Bicycle Coalitions: http://www.ebbc.org/safety
If not, check with your local shops, riding clubs, or bicycle coalition to see if anyone offers something similar.
UPDATE: I just wrote a new post specifically for new riders who aren't used to being in traffic, to help you avoid getting hit by a car.
This post has been way more popular than I ever expected, and since it is intended for new riders, I thought it would be pretty important to help y'all not only pick out a new bike, but not get run over while you are riding it!
Read this before you get on the road: http://biodieselhauling.blogspot.com/2012/06/please-ride-your-bike-in-street.html
*** I won't pretend I stop at stop signs, or even try to convince you to. But at least slow down for them, and look both ways before you cross. And always come to a full stop if there is cross traffic which has the legal right of way.
[Someone has written an article with counterpoints to this one. Personally, I disagree with him on a few points (1st off, that you should never buy a used bike!), but it is always worth getting 2nd opinions and different perspectives: http://hiawathacyclery.blogspot.com/2012/01/bike-buyers-guide-for-beginners.html