This is the second water drop tutorial I've featured.
The first was done in a paint program using the applications that were available to that particular program.
This one is done strictly in Photoshop, using only two brushes.
~Remember, your water drop will use the colors of the background it rests on!~
Step 1: Using a hard brush, pick a darker color of the background color your water drop will be resting on, and draw in the basic shape of your water drop.
Step 2: Still using a hard brush, next pick a lighter version of the background color.
*Important* - The general rule is the light source determines where the refracted light, or lighter part of the drop will go. Here the light source is coming from the upper right, so the lighter color will go opposite of the light source, hence, down in the lower left of the drop.
Step 3: Using a softer brush (I use the airbrush brush with slightly reduced opacity and flow) blend the light color into the darker color. Use the actual background color as needed.
Step 4: Back to the hard brush again. Using a much whiter color, add the highlight. The brightest highlight goes over the darkest area.
Using the dark color, add a small shadow to the lower left outer edge of the drop, if you want.
Again, this is a "standard" water drop, but looks pretty cool when it's applied.
Good luck and have fun.