Currently Being ModeratedFeb 7, 2013 10:19 AM (in response to sandrashriver)
As you have a 27-inch Late 2009 iMac, you have got a 1 or 2 TB hard drive. As you have a big hard drive and a small amount of free space, get a 2 TB hard drive minimum that you can connect with USB or FireWire (I recommend FireWire because it's faster).
The default way to back up your Mac is using Time Machine. This will create complete backups of the hard drive onto the external drive. However, Time Machine doesn't create bootable backups, and you may need it if your hard drive gets damaged, so you should use Carbon Copy Cloner or SuperDuper on a different partition or external drive.
Also, I can see that your most important files are your music and photos. As you have a small amount of free space, you can delete some folders from the Time Machine backup to save space onto the external drive: open System Preferences > Time Machine > Options, and add folders that you don't need to Excluded Items.
For more information > http://pondini.org