Hi, basically you need a transparent container, a black backdrop, one of two wireless flashes, water, and lots and lots of of shots.
1) make your set up- place the object on a black or clear plastic box which acts as a stand. place the whole setup in a tray to catch the water, position your camera on a tripod and prefocus manually on the object you are dropping water on. if you are dropping things into water, use a transparent plastic or glass container, then put a ruler in where you think the objects will fall, and prefocus manually on a tripod. [rmb to use UV filter or plastic bags around the body to protect your camera]
2) Set up your wireless flash/es to the side of the setup, experiment.. diff set ups/objects are best lit from different directions, but off to the side (as opposed to front- or back-lit) makes the water look the best.
3) shoot on manual and use a small aperture like f7 or f12, keep shooting until you find a nice position for the lights/object, then continue shooting to get a nice array of droplets.
it really takes a lot of trial and error, because water will do as it pleases. use photoshop to clean out the stand if it is in the picture, and experiment with e.g. coloured backdrops to make interesting patterns in the water droplets.