Today I am going to start out with something pretty basic. The aperture is one of three of your cameras main controls. It controls how much light comes in through the lens and the images depth of field. (how much is in focus). When I want a certain effect from using a certain aperture I usually will set my camera onto Aperture Priority. This allows me control of any of the creative settings I want while automating the shutter speed to expose the picture correctly.
With a larger aperture (which is actually the smaller numbers) your image will only focus on a small portion of the picture and throw everything else out of focus. With a small aperture (the bigger numbers) it will tend to make the whole picture in focus. I should also mention that how close your camera is to the subject or your zoom can also effect this.
This picture was shot at a very large aperture, 4 I believe, And I was very close to the subject causing the blurring effect. There are some blondies that Melissa made.

This is an example of a smaller aperture of 22 where the whole scene is in focus.

Now here is what I am really trying to work on right now. Varied levels of focus at different depths of the image and the focus plain being somewhere in the mid range and not just in front. When I first started playing with this I shot a lot of things like the blondies above with the subject front and center with the background blurring out. This is still a good effect in many instances. I am trying to work on putting my subject somewhere in the mid range and blurring out the foreground as well and the back ground. Here are two examples below.
Shot at the peace gardens in Slc, Ut.

Shot at Weeping Rock in Zions. I though it was cool the flowers were growing out of the wall.