You often hear or read that to achieve a shallow depth of field you need a fast lens, i.e. one with a large aperture. Like a f/1.4 or f/2.8. This is true in a lot of situations but not all. The depth of your focus field depends very much on how far the subject you are focusing on is from your camera.
Say I'm using a 50mm lens:
f/4.0 50 mm 1/100 s ISO100
I could use an aperture of f/4 and photograph two models with their head more or less next to one another. If I stand about 6 feet back I'll have them both pretty much in focus.
f/8.0 50 mm 1/60 s ISO320
But if came in closer, wanted to fill the frame and came up to about 2 feet; I'd focus on the front model, then the one behind would already be slightly out of focus, even if I stop down to f/8.
So keep the distance to your subject in mind when you are choosing your depth of field via your aperture setting. If your camera has a DOF preview button which stops down the lens so you can see the effect through the viewfinder use that get a feeling for the needed aperture.