The Google Fit repo on GitHub has code samples explaining how to use the Android APIs. There are also code snippets for other common tasks in the developer guide.
Read data from sensors
GitHub code sample to register listeners to read raw sensor data from a data source.
Record data to Fit storage
GitHub code sample to request automated storage of sensor data by creating subscriptions.
Read data from Google Fit
GitHub code sample to read data from the user's fitness history.
Work with sessions
GitHub code sample to demonstrate sessions: Encapsulate fitness data as a logical session, to represent an overall workout.