What I do is look at what settings the camera is choosing in auto mode, pay attention to the light meter (close to 0 as possible), and if I want to adjust something myself, I do it in either shutter or aperture priority mode.
A lens with a higher aperture value (lower f-number) means it's 'fast'. In other words, it can work better with higher shutter speeds, and return, in lower light. This is why lenses with lower f-numbers are usually more expensive. If you 'stop down' this means you're setting the f-number higher. The lower the number, the shallower the depth of field. For example, two pictures taken at the same focal length will be different at f/2 and f/8. The f/2 will have a blurrier background and foreground while the f/8 will have everything in focus. (there's a good illustration of this here:
Depth of Field: Optical: Glossary: Learn: Digital Photography Review ) So if you find that you want more bokah (blurry background) put the camera in aperture priority mode and lower the f-number. Experiment with getting closer and farther from the subject, different focal lengths, etc.
You won't find youself adjusting shutter speeds often unless you want a specific amount of exposure from a photo that the camera doesn't do in auto mode - like a night shot that you want to see trailing car lights in. Or if you're taking pics of a moving car and you don't want it to be blurry. If you adjust in shutter priority mode, the camera will choose the best ISO and aperture settings so you won't have to worry about them.
Shutterspeed: Exposure: Glossary: Learn: Digital Photography Review
Hope this helps!