How to encrypt your entire device (computer, tablet or phone) depends on the kind of device and the operating system:
NOTE: If you encrypt your device, your information is only encrypted when your device is powered down. It does NOT protect your data if someone gets your device while it is powered on. Therefore, you must always always always turn off your device before situations where it might be seized (like a protest), and remember to turn it off in unexpected situations where it will be seized (like a police raid.