The easiest way for you to do the above advice is to stop your lens down. If the waterfall is in a bright area, it's going to be more difficult, especially if you have a prime lens. You don't want shutter priority, you want aperture priority for this:
Load 100 ISO film or set your digital camera. Set your camera to AP, put it on a tripod, use a shutter release / remote control. Compose your shot with care - look out for scrub, leaves and distracting stuff. Put on a UV and/or polarising filter, or a ND filter for colour. Use the longest prime lens you have or longest zoom, and stop it to f22 or whatever your maximum number is. Focus it and have another look at your composition. Get up close and have a look at your subject - usually an interesting shaped rock. Remove leaves, twigs and debris.
Take your shot and count the seconds of exposure. I find that over five seconds will give you mist and glass effect; 1/2 or one second will give a touch of movement, but still glassy; 1/15 is my favourite as it depicts movement, but smooths the river/stream. It's all about your idea - start at f-big number and work down to f-low number.
I have some pictures of the sea and the Thames on the site, and Thelma has a waterfall picture on the site. Thelma's waterfall is taken at "normal" hand-held exposure, i.e. above 1/30th of a second, so there's movement, but probably not near the effect you are trying to achieve.
Good luck and post your results!!
Rob