Please note that this is not meant to be a discussion of backup for business,
which entails planning, policy, maintenance and testing and will differ widely from organization to organization.

With any luck, you've worked with the structure that OS X presents you, and you've kept your files in your home-folder: the folder with the little "house" icon, which will be named according to the shortname you specified when installing your OS (do not try to change the name of your homefolder - it's not as simple as that). However, if you like to spread your personal files throughout your hard drive, then you'll need to track those down.

You can backup your entire home folder, which will cover all of your files (see caveat above), and your personal settings.

It's not "officially" possible to backup Applications, and many programs will have numerous files spread out throughout the system. The first of which is often:
/Library/Application Support

You should backup as often as you feel comfortable with, a good median is once a week, but this can change according to how often you change your documents, and if (say for example), you have a vital project you're working on (backup each day's work !).

Please don't be lulled into complacency about backing up, there's simply no substitute for regular backup of your precious info.

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