Ahh, you are trying to get the "silky waterfall effect."
In general, you need to get the shutter speed slowed down to something in the range of a large fraction of one second, like 1/8 or 1/4 or 1/2. It may be even more or less, and it depends on the water height and distance. Now, if you shoot that handheld, it is likely to be a mess because of user jiggle. If you use a tripod or something else to stabilize the camera, that is almost a necessity. With some cameras, you still can't get there from here. With some cameras, it is difficult to shoot at 1/4 because there is too much light. In other words, it can't get the aperture to stop down far enough. So, you need to help it. Step 1 is to manually set the ISO for the lowest number possible, something like 50 or 100. Step 2 is to manually set the 1/4 shutter, and you use T mode for exposure. If those work and you can shoot it, then fine. If the camera won't let you do that, then you need one more step, and that means adding a neutral density (gray) filter on the front of the lens. This ND filter will knock out one or two or four stops of light, so that makes the earlier steps easier. I'm guessing that you do not have any ND filter.