you just need a camera, and some objects to put in distance.
I'll use a scene I've already setup and 3 spheres.
My rendered image is looking like this:
Open up your render settings, and select Depth of Field.
Now add a camera
With the camera selected, Click on that button to open up all perspective views, just like this:
Now, you can see the camera pointing at your spheres. And now, we have to activate the depth.
With the camera still selected, look on your lower right corner, and select the "Depth" tab.
The depth of field, basically blurs the visual, the front visual, and the rear visual.
Front means, in front of the focus point, and Rear means behind the focus point, in this case, I'll check both blurs.
Now Let's change the focus point and the blur points, focus on the "Top" perspective, as you can see, the camera has some more green lines, those are the depth lines. The one in the center, is brighter, and it means that the camera has the focus point over there;
Drag that bright green line, over your focus point, in my case, the sphere on the center.
The other darker green lines, must go over your distance points, in my case, near the 2 other spheres.
Now press again the button to close your perspectives, and try to quick render