Linux full disk encryption
Encryption is best enabled on a Linux system at build time. Noawadays, many current distributions present this as a "checkbox" option in the install wizard, usually using LUKS disk encryption.
Unlike Windows and macOS, enabling full disk encryption on an existing Linux system is tricky, and it's generally easier to make a backup and then reinstall from scratch.
However, for anyone brave, here's a guide on encrypting an existing Linux system.