First thing you will need to do is use the Pen Tool to create a rain drop shaped path.

Make a simple triangle then use the Convert Point Tool to turn the triangle into a rain drop. Don't worry about making the perfect rain drop just get a decent outline.

When you have a decent rain drop goto Edit > Define Custom Shape... and save your shape. Now it should be stored with all the other custom shapes. Switch to the Custom Shape Tool (U) and click on the Shape drop down and select your rain drop.

Set your foreground color to white, create a new layer, then draw as many rain drops as you want; they can be any shape or size. Once you have your raindrops, open the Layer Style window and edit the Color Overlay and Inner Shadow. I used the exact same color for both effects then adjusted the opacities until I was happy.

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