Backup Drive
The first thing you'll need to use Time Machine to backup your Mac is a second hard drive.
Time Machine works great with external USB or Firewire hard drives. Generally you want a drive that is at least twice as large as the amount of data you are backing up. For instance, if your internal hard drive is 500GB and you are using 250GB of on the drive, the minimum size drive you'd want to backup to is a 500GB drive. However, keep in mind the amount of data on the drive can easily increase so I just go by a rule of thumb and get a backup drive that is at least twice as large as the drive capacity of the drive you are backing up. So a 500GB drive could backup to another 500GB drive, but as the amount of data on the drive increased, there would be less room for backups on the external drive. With a 1TB drive (1000GB), there would always be plenty of room for backups.
To find out how large your internal drive is and how much space is being used and how much is available, click on the internal drive icon on the desktop (often it's named "Macintosh HD") and then click the Command and "I" button at the same time to "Get Info".
You should see a window like the one below. In this example, the internal drive is 250GB and just over 20GB are being used.
