I started with aperture priority (A on a my Nikon and AV on her Cannon). My quest for acquiring more photographic knowledge began with trying to figure out how to blur the background. I now know that the blurry background is called bokeh. The aperture is controlled by f-stops. I am not going to explain this now, other than to say this:
The smaller the f number (e.g. f 2.8) = less of the photo in focus (i.e. creates the blurry background/great for portraits):
The larger the f number (e.g. f 16) = more of the photo that is in focus (great for landscapes).
By switching to aperture priority mode, you control what is called, 'depth of field'. This is the information that I gave to my friend. We had a great time and I was able to capture a few shots for myself!