The Sierra Club has some good, practical tips for getting started with bicycle commuting:
Join Forces. Is there a bicycle club in your area or, even better, a bicycle-activist organization? Getting together with other cyclists is a good way to learn about routes, improve your skills and technique, and find fellow commuters. There's greater safety in numbers.
Plan Your Route Carefully. Don't assume that the shortest, most direct route is the best - quality beats brevity. Look for quieter streets that aren't packed with buses and trucks. If you must ride on busy streets, use those with bike lanes (if you can find any), or at least wide lanes. Multiple-lane, one-way streets are often good. And stay off sidewalks - it's usually illegal and always dangerous.