First off, let's begin by saying that claiming a home office is useful because you can claim items -- or a portion of items -- you wouldn't normally be able to claim for exemptions. (Think that Internet bill, or even the office chair you bought.)
One big rule: You need to use your home office exclusively as your principal place of business. That means that you can't claim the 4 feet (1.2 meters) surrounding the couch in your living room as an office space, because obviously you're not exclusively using it for business. (Unless your business involves binge-watching "House of Cards," in which case I'd like to give you my resume.)
So got that? You don't necessarily have to use an actual room of your house only and exclusively for work, but the work area should have some clear demarcations.