How can I send a private message to someone on Facebook?

To send a message:

1. Click Messages at the top of the screen.
2. Click New Message.
3. Start typing a name into the “To” field. Names of friends will appear in a dropdown list.
4. Select the person or people you want to message.
5. Type your message, then press enter to send.

The following link will also give you more detailed instructions:
https://www.facebook.com/help/154917244574299