With such a low shutter speed you won’t be able to hand-hold your camera. A sturdy tripod is an essential accessory here.
Set your ISO as low as it will go (typically around ISO 100 or so). This reduces your camera’s sensitivity, allowing you to use slower shutter speeds without overexposing the scene. It also has the added benefit of reducing the amount of digital noise in your photos.
Using your lens’s narrowest aperture will again let you use a longer exposure time. It will also give you the maximum depth of field, keeping as much of your scene in focus as possible.
If you still can’t get your camera to go slow enough you’ll need to use some filters to reduce the amount of light that’s being let in. Professional nature photographers swear by neutral density (ND) filters, which reduce the light without affecting the colours in the scene.
An excellent alternative is a polarising filter. This does the same job as an ND filter but has the added benefits of reducing reflections (for example from water, wet rocks, and leaves) and increasing colour saturation for a more vivid image.
SHOOT AT THE RIGHT TIME OF DAY
Bright sunlight can easily ruin a waterfall photograph. The intense light casts strong shadows across the scene, making it difficult to get your expsosure right. It also causes hundreds of reflections in the water and wet scenery, which will show up as tiny white dots in your shot.