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Aperture
Think small numbers as a big hole and big numbers as a small hole. Aperture is directly related to depth of field, or how long of a range (distant from you) that will be in relatively crisp focus. Small apertures have small depths of field and are useful for getting the subject you want in focus as the only thing in focus.

shutter speed
There's a concept called exposure which is a combination of shutter speed and aperture. As you decrease one you need to increase the other in order to let the same amount of light hit the film (or the CCD for digital). There are a multitude of combinations giving you the same amount of light, but with the different apertures you get a different depth of field and as you fiddle the shutter speed, you can stop motion, or blur motion. The best motion photos for waterfalls tend to be duskish or overcast days when you can close down the aperture (high number) and get everything around the subject in focus, but then still slow the shutter speed down to sub 1/30 of a second and still not overexpose the photo.

washed out skies
You could use a polarizing filter to help reduce the refracted light in the sky, but another method is to spot focus on something midrange in the sky and take your photo with that setting. If you have exposure compensation, just dial it down to shoot minus a half to a full stop below what the camera is thinking.

Just try as many different combinations and find what works for you. I tend to shoot aperture-priority mode so the camera deals with the shutter speed for me and I just determine what I want for depth of field (aperture) and watch the shutter speed in the window, then use exposure fiddling to bump things down 1/5 to 1 full stop below what the camera is thinking.

And I'll go against brickell (no malice intended) and say set your ISO to the lowest number your camera will do. You'll thank yourself later. Unless you are trying to stop motion you'll get crisper photos out of lower ISOs.