The American Cancer Society suggests how to approach the subject:
Compose a list of people you want to tell yourself, and other people you want them to notify for you. Also, jot down thoughts about how you’d like to begin, and what you want to share.
Explain your type of cancer, and share as much about your diagnosis, prognosis and treatment options as you are comfortable with.
Encourage family and friends to do research on their own.
Think about any topics that are too difficult for you to discuss, and avoid those topics.
Compose responses to likely questions, and know when and how you’d like to end the conversation.