To secure a computer you should enable FDE (full disc encryption) on the device.
It is likely that FDE is already enabled. To check, navigate to System Preferences -> Security & Privacy, and click on the FileVault tab.
If it is not enabled, enable it by following the instructions on the page. This will take a long time to complete.
TODO: fill this in
- How to encrypt a Windows computer
- Same for Linux and MacOS (why not, they're easy)
- How to password protect these computers
- Possibly a discussion (either here or linked from Securing Android devices) on how data gets stored etc