If you have never had a hard drive crash consider yourself to be blessed. Consider all that you have stored and if you would be willing to throw it away. If you do not have a backup of your hard drive it will be a very hard lesson to learn when it happens. Click on the appropriate link below for help on backing up your particular computer.
When backing up you should do this to an external drive. Most drives are fairly inexpensive, the key is making sure it will be large enough to handle your drive. The important point that I cannot stress enough is that once you backup you need to disconnect the drive and store in a secure location. That way if your computer is stolen they do not get your backup drive.
As to how often you backup the computer is dependent on how much work you are doing on your computer. If you work on your computer daily and are generating several documents/files a day than you will want to backup at least once a week. If you are real heavy on the number of files you generate you may want to backup every other day.