A picture of the upper falls at the Blue Hole outside of Elizabethton, TN. What's great about the Blue Hole is that you can see four beautiful waterfalls in a short amount of time -- and each of the falls is unique from the others. See my original post for more discussion and for directions.
Photographing waterfalls is not as difficult as you might imagine. You need the following to be successful...
1. The right support. There's no way around this, you need a tripod. You can buy a cheap $30 tripod at Wal-Mart (which is where I started). Eventually, you'll get nervous about setting a thousand dollar camera on a $30 tripod in the middle of the creek and you'll step up to a better one. A heavier tripod, besides giving you peace of mind, will also give you a steadier, less shaky camera -- which will produce more crisp photos.
2. The right light. A cloudy day is best for shooting waterfalls. Bright sunshine is not helpful when you need to leave the shutter open in order to get the silky-water effect. The sun will produce glaring 'hot spots' in the image which contain no detail. The other option is to take your waterfall photos with early morning or late afternoon sunlight -- this warm light produces some of the most beautiful photos. The point here is that your light needs to be even and diffused.
3. The right settings. There is no one setting that will work in every situation. The amount of light and the amount of water flowing over the falls are both factors. Having said that, I'd recommend when starting out that you put your camera on shutter priority ("S" for Nikon and "TV" for Canon). This means that you will set how long the shutter will remain open, while the camera chooses the appropriate aperture for the lighting conditions. A good rule of thumb is to start a one-second shutter speed. If the scene is bright, you'll need to leave it open for less time (try half of a second). If you're in a dark area or if you want a greater depth of field, it will need to be open longer.
4. The right position. What I've learned (and what I constantly need to remind myself of) is to not set up the tripod too quickly. Instead, take time to survey the situation. The best position to photograph a waterfall is not necessarily directly in front of it. For added interest consider various angles. Next, consider various heights. Setting the tripod up at water level can make for a neat effect (You might want to check your homeowner's insurance to see if it covers your camera taking a dunk in the creek!!). Finally, consider the background. Oftentimes, it is the context sounding the waterfall that either enhances or detracts from the waterfall.
5. The right filter. A circular polarizer which threads on to the end of your lens is a must for shooting any type of water scene. With a turn of the polarizer, you will see the harsh and distracting glare disappear from the surface of the water and even from the leaves of the trees. I've recently learned that it's not always necessary to turn it all the way to render the water black. To show movement, keep just a little glare on the water.
I hope these tips and recommendations will help get you started. The best thing you can do is to shoot lots and lots of pictures. Experiment with different settings. Digital photography makes this easy. Unlike film cameras, it costs nothing to press the shutter release on a digital camera. Canon has a terrific website that discusses how to use a digital SLR camera. Even if you don't shoot Canon, I think you'll find this to be a good resource, especially if you're just starting out. (Here's the Nikon 'digitutor' website).