If you're not parking a car, you won't want to buy a smarttrip card - you'll want to use the one pass or day pass paper (and disposable) cards. There are machines in every station (they take cash or credit cards). Note that you don't get a refund, so shouldn't overfill the card too much.

SmartTrip cards are more for the locals and regular communters and for anyone parking a car (since the parking lots are unmanned since they realized the folks working those gates were stealing metro blind).