John often uses his computer around his roommates.

Open System Preferences and click Desktop & Screen Saver. Click 'Require password immediately after sleep or screen saver begins.'

Open Security & Privacy in System Preferences and click 'Require password immediately after sleep or screen saver begins.'

In System Preferences, click Energy Saver, then click 'Require password immediately after sleep or screen saver begins.'

Move the cursor to the bottom-right corner of the display and enter a password when prompted.