When we initially look at all the settings and control functions on a DSLR camera one could get intimidated.
This is the Control Mode dial on my Canon EOS 60D:
The Green Box represents the Automatic setting. here the camera automatically sets shutter speed, aperture, ISO etc based on the available light. Most point and shoot, aka pocket cameras provide automatic, manual, portrait, night, scenic etc settings with very few if any operator settings.By switching modes you can expect to get a particular image darker, or lighter and which is more satisfactory than just shooting straight on automatic. Change the settings, preview the results ..if its not to liking, just delete it and take another shot.

The image above and left was shot...(shot or shoot is photography lingo for taking or capturing a picture)...Ex: We are going on a photo shoot this Monday) in Manual mode. Shutter speed of 1/5 second, aperture of f5.0 and ISO of 100.  As you can readily see this image is too light, or over exposed. The result could be changed by changing the shutter speed, aperture and/or ISO. For example: shooting this same scene with a shutter speed of 1/125 or 1/250 of a second,and aperture of 5.6 or f8.0 would render more vivid colors and detail.

Here we changed the Mode Control dial to the "mountain with small cloud" icon which is Canon's "Landscape" setting.This image was shot at: Shutter speed of 1/100 at f5.6 and ISO of 400.

For this image we changed the mode dial to Aperture priority (Av). Here you set the aperture such as f5.6 or f8.0 and the camera automatically sets shutter speed, ISO etc based on light levels coming through the lens and focused upon the sensor or the film plane IF you are using film. The settings for this image: Shutter speed of 1/100 of a second, f8.0 and ISO of 800. You can see a difference in the brightness and detail of the mountains and clouds in this image when compared to the Landscape setting image above..

Now, we will show you some bracketed images taken in Program (P) mode. In "P" mode the camera sets shutter speed and aperture. In these bracketed images, one was at normal "P" setting and one is taken at -2 stops under and one at +2 stops over the normal setting.This can be set in increments of 1/3 stops up to 3 full stops of aperture. Most DSLR's support this, point and shoot models do not.

This image taken in "P" mode, shutter speed of 1/100 of a second, f5.0 and ISO of 250.

This image was shot in "P" mode, shutter speed of 1/125, f5.0 and ISO of 100.

This image shot in "P" with shutter speed of 1/100 of a second, Aperture of f5.0 and ISO of 1000.

So we have some examples here of how even small increments in shutter speed, aperture or ISO can effect the final image. Hope this helps in your understanding of your camera functions and if you have any questions please reply.
Keep on shooting images.