Water droplets can be created in Photoshop largely with layer styles, but there are a few other key techniques that really sell the effect
First thing is to simulate the magnification effect of the water. So begin by drawing a drop shaped selection.
Copy that selection of the leaf to a new layer (Cmd/Ctrl+J) and scale it up (Cmd/Ctrl+T). Intensify the colour with a Hue/Saturation adjustment.
Begin with a Bevel & Emboss layer style with a Depth at 200%, Size at 193, Soften at 4. Change the Angle to -117 and Altitude to 80. Reduce the Highlight opacity to about 50%.
Add an Inner Shadow with a Distance of 12, Choke of 27 and Size of 21.
Inner Glow with the colour set to a pale green, Choke of 7, Size of 50. Outer Glow set to Multiply, colour to a dark green and Size of 18.
Finally, a Drop Shadow with Angle of 66 and Size at 32.
All of these settings are unique for each drop, so use them as a start point and find the combination that best fits the image you are working with.
Add a layer above the drop and use a small brush to hand paint in the white highlights.