Tap Water Effect

What you are trying to make is a composite. At the most basic level, a composite is just layers. To make water run from a toy tap you will need a layer containing the footage of the tap and a layer for the water. Depending on the angle of the shot you may need to do some masking to trim the water stream so it looks like it is coming out of the tap. Sometimes this is easiest to do with a track matte so that adds another layer.

If the camera is moving you will also need to do some motion tracking or motion stabilize the shot and then put the camera moves back into the shot and tie the water stream to the camera movement. which can add additional layers.

As Roland and Andrew have pointed out there are different ways of creating the stream of water. You can film an actual stream of water against a black background, adjust the levels and use a blend mode, you can film against a green screen and key out the stream but that requires some extremely careful lighting and a really good to do well. You can simulate a stream with everything from Fractal Noise + distortion effects to Trapcode Particular. There are also many ways to do water simulations using 3D software. Blender (blender.org) is a tremendously powerful open source 3D app that you can use to create the water stream and even do the composite.

All of this requires planning. To give you a step by step we need to either have a detailed description of the shot you are working on or better yet, see the shot. We also need examples of the kind of water stream you want to create.

If you are really interested in compositing and visual effects I suggest that you spend some serious time studying the process. There are many excellent books on the process. It really does not matter what software you use to assemble the final composite, the planning and the process of preparing the footage and layering is all the same. Let us know if there is anything else we can do to help you.