Sign into your Google account via Gmail (or any other Google service)
Go to your Google profile here: http://google.com/profiles/me
Click the link at the top-right of the screen that reads "Edit Profile"
Here, you'll see a checkbox that reads "Display the list of people I'm following and people following me." To make this info private, just uncheck that box.
Scroll to the bottom of the page and click the "Save changes" button