how can i backup my HD?

There are any number of backup apps available. Just make sure you have enough space on another hard drive. If you want a true backup (off-site), look for a app like Carbonite.

If you want to clone the hard drive (including hidden system files, etc) to make a bootable disk, use an app like SuperDuper or Carbon Copy Cloner.

-DH

Have you tried Time Machine? It really works very well. Get an external drive that works with your Mac, plug it in, then go to the Apple logo at the top left of your screen, and click it. Choose:

>System Preferences>Time Machine

and select your external drive. Backups automatically follow.