While I'm not a long exposure person, I've gleaned a number of things from various posts/threads on this website.
For waterfalls, oceans, etc, the way to get 'smooth' water is a longer exposure...roughly 1/2 second or in that range. Of course, a tripod is REQUIRED for these slower shots...or at least a stable, non moving platform of some kind. A wireless remote control is also recommended, as even your finger leaving the shutter button produces a slight camera movement.
Lowering the ISO in your camera to 100 and a smallish aperture (f22) is a good start to getting an exposure of 1/2 second. It may also require a neutral density filter simply to 'darken everything', thus requiring a longer exposure. How dark of an ND is required is the subject of a number of threads here as well.
Rather than starting with moving water right off the bat, it's far easier to go out at night and photograph some buildings, bridges, etc, under various lights. Some may be very dimily lit, others, like a Walmart, would be very brightly lit up. 35 years ago, I even did some night shots of downtown Los Angeles from the mountains. With 'fast film' of the day (ASA 200!), I used a cable release and took a number of pictures from about 2 minutes to 5 minutes long. "Spray and pray" with film...also called 'bracket exposures', then picked the winners after the slides were developed. Exposures that long with digital require some heavy duty noise reduction processing. If done in camera, it's measured in MINUTES of 'camera busy' after the shutter has closed. So it's better done in post processing to save time.
In short, it's practice, practice, practice. Fortunately, these days, we have the EXIF data on each photograph, so we can go back and see what settings worked and what didn't work and improve the keeper rate.
