Dof is simple. Lets say you have your camera at 0 and you have your main object 1 meter from the camera. Now go to render tab and the go to Blur tab. Set the Focus distance to 1. Then the more u crank the Aperture diameter the more the background will be blurred. Meaning the main focus will be at 1 meter and the rest behind will be blurred. Or if you have an object at 0.5 meter then that will be blurred too since it is out of the focus.
(Before you render make sure you clicked on the Dept of Field under the Render tab.)
Hope it helps.
Btw render time will increase.