The US CDC have a useful feature on the basics and importance of hand washing.
Some of the information presented includes:
"Keeping hands clean is one of the best ways to prevent the spread of infection and illness.
Handwashing is easy to do and it's one of the most effective ways to prevent the spread of many types of infection and illness in all settings—from your home and workplace to child care facilities and hospitals. Clean hands can stop germs from spreading from one person to another and throughout an entire community."
"When should you wash your hands?
Before, during, and after preparing food
Before eating food
Before and after caring for someone who is sick
Before and after treating a cut or wound
After using the toilet
After changing diapers or cleaning up a child who has used the toilet
After blowing your nose, coughing, or sneezing
After touching an animal, animal feed, or animal waste"
To see more and to see some good illustrations, go to the CDC page.
After touching garbage