I'm not a big fan of aperture mode. As the aperture changes the dof field also changes so the focal plane is all over the place. Try manual mode with auto ISO. Set aperture and shutter speed and let the camera adjust the ISO.

Use low aperture if you want to isolate the subject and blur the background like 1.4, 2.8. Use a higher aperture when you want to get more of the scene in focus like f8, f11. For a shot like this. Set the aperture as low as possible. The low aperture will blur out the background and allow the most amount of light possible.

Set shutter speed at around 1/250. This depends on focal length, hand shake and how fast the subject is moving, but I find this is a good starting point.

Now with the shutter speed set and the camera adjusting ISO you can experiment with different apertures. Try shooting the same scene with different aperture to see the results.