Someone can help me with a tutorial about water drop effect on body car?

You can find a lot of rain drop brushes and you just brush them on a layer over a car, play with transformations, follow the lines of a car, wrap it, distort it, play with opacity.
You can use png images like these: http://pngimg.com/uploads/rain/rain_PNG13473.png
http://www.autemo.com/dc/users/3/SRFY6/6789/chops/NISSAN_skyline_r34_by_Nihad08.jpg This is my old chop where I used that image, look how drops follow the lines of a car, take a look at the bumper or fenders, hope it helps

Cheers my men! Will help me so much!