Time Machine is a program for file backup that comes with your Mac. It works by creating incremental backups of all your files on your computer so they can be restored at a later date should you lose anything.
Time Machine also allows you to restore your whole system should your computer fail to start.
Connect your external hard disk to a USB port as shown here.
If you haven’t yet specified a backup device for Time Machine to use, the program will ask if you would like to use the external disk for backups the first time you connect it. Click "Use as Backup Disk," and you’re done.
Confirmation is shown in ...