Enabling encryption on your macs harddrive is easy.

goto system preferences > security and privacy > filevault

click the padlock in the bottom left hand corner
type your password
click ‘turn filevault on’

you will then get a recovery key.
you can write this down, print it, email it, whatever you want.
note that if you want to you can send the key to apple and if you forget your password you can phone them and recover your mac.

when you have turned on filevault your mac will restart and the encryption will begin.

you can check the status of the encrypting by going back to the filevault settings in the security and privacy section in the system preferences.

you can get a full list of the options for filevault from this apple help page