Lots of good advice so far, especially the pointer to the link for Sheldon Brown.
Join a local bike club to learn, and check out randonneuring
http://www.rusa.org/
Bike fit is extremely important for rides that last more than a short time.There are several different ways in which the bike can be adjusted to fit you. Get it adjusted at your local shop, It's worth it.
Find a good local bike shop with a good mechanic. Ask about good bike shops at the bike club meetings you attend.
Ride with traffic on the right hand side of the road. This is important because cars turning on to the road at intersections will not see you coming if you are on the wrong side of the road. They just don't look for traffic coming from the wrong direction.
Always pass other cyclists on their left, and call out "ON YOUR LEFT" in a big loud voice. It avoids collisions. Don't be offended when someone says it to you. Its the safe thing to do.
Be extremely cautious about passing cars. Cars changing lanes, or turning right at intersections or driveways tend not to see cyclists and you can be caught in a "right hook" accident.
Parked cars can be dangerous when a person opens a door as you pass. Then you get "doored." It hurts. Don't drive close to cars parked along the road.
Don't use a soft bike saddle. Get a firm one. I know this is counter-intuitive, but ask around at the club, making sure you are talking with people who are actually riding, not rookies who are thinking about it.
But my number 1 tip for beginners [drum roll]
If you are steering to avoid a rock or pot-hole in the road, focus your eyes on where you want to go, not on the pot hole. It works like magic to help you steer around things.
Last but not least, remember to keep the rubber side down and the shiny side up.
Have fun.