Similar to any athletic activity, you need to train and practice to ride safely. To help get you started, here are some tips:
Learn how to ride in a group-You can save 40% of your energy by drafting the rider in front of you.
Look for group rides in your community or check with your bike shop.
Always know what is happening behind you at all times. You can check over your shoulder several times or simply invest in a bicycle rearview mirror. A small helmet mirror works well after you get used to it.
The most beneficial training for the ride is steady aerobic exercise. Ride on gently rolling roads offering 5-10 uninterrupted miles.
Train at the aerobic level by riding at a "conversational" speed. When riding, you want to be at a pace that is ok for you talk but more comfortable if you are not talking.
Before and after any type of exercise, stretch the muscles you'll be using. Stretching prior to exercise helps to prevent injury.
As always, be sure to consult your physician before beginning any new fitness routine.