That effect is a result of a shallow Depth of Field (DOF). To get a shallow DOF, you can a) use a wider aperture and b) use a longer lens.
So with your gear, the 50mm F1.8 will be much better for this because it's maximum aperture is much larger (F1.8) than the kit lens's max aperture of F3.5-5.6.
So first thing to do, would be to put the camera into Av mode (aperture priority) and set an aperture of F1.8. In this mode, the camera will give you a shutter speed for the aperture that you have chosen. Now go out and try it. You may even find that the DOF is too shallow...and the whole person might not be in focus. If this is the case, stop down the aperture a few steps, maybe to F2.0 or F2.8 for example.
Also, just how out of focus the background is, will depend on how close the camera is to the subject and how far away the background is. For example, if you put a person right under a tree and stand back 30 feet...both the person and the tree will be in focus. But if the person is 25 feet away from the tree and you are still 30 feet from the tree (5 feet from the subject)...then the tree will be out of focus when you use a large aperture.
One thing to watch out for, is that you are focused on the right thing. Sometimes the camera may want to focus on the background, which would leave the subject out of focus. For this reason, I prefer to use only the centre focus point. I focus on a person's eyes with the center point, lock the focus, compose the shot how I want, then shoot.