How to make it so your long exposure photograph doesn't turn out white?

I have a Nikon D3100 and i was at a waterfall not too long ago and i tried to take a long exposure photograph of the water. However, when i took the picture… The whole picture was white. I tried different settings, but i couldn't find any that it works with… Does anyone have the same problems? Can someone help me?

You should use a small aperture, and if that doesn't keep it from being overexposed you will need to use a neutral density filter to cut down the amount of light that travels through the lens.

To achieve the silky effect of a waterfall you need a Neutral Density (ND8) filter, low ISO and small aperture.
http://www.cameratonic.co.uk/hoya-58mm-nd8-nd-8-neutral-density-filter.html

"Neutral density camera filters (also known as NDs) produce a grey neutral tone and are used to reduce light in photographs, enabling more control over exposure and depth of field without affecting colour or contrast. They are especially useful when you are taking photos in bright light conditions to avoid overexposure. Since our and camera filters uniformly reduce light at all wavelengths, there's no effect of the colour rendition."

Have a basic understanding of exposure.

You either didn't stop down enough, didn't use a low enough ISO, or you should have used an and filter.

You clearly do not understand the first thing about creating a properly exposed photo. You should be ashamed of owning a DSLR and not understanding something this basic.