first of all: I'm not too experienced with AE so it may be an easy task but basically what I wanna create is like a "Coin Rain" - I have a small Video file with an animated coin created from a gif (it's an pixel art coin, like the one's you know from Super Mario etc), so I just need the "falling down" part. I am looking for a way to easily create it, sort of automatic multiplying and randomizing of the position so that the end result is a 5-10 second long stream of coins falling down (some should be a bit faster, some a bit slower, so that it looks more real and appealing)
Hi Max, a particle system will do that sort of thing. To quote Kevin Camp in another post:
create a new solid and add cc particle world. in the particle settings, set the particle type to textured square, and then in the texture settings select your GIF as the texture layer and set the texture time to birth. a little lower in the particle settings, set the birth and death color to white (this will prevent particle world from tinting your images).
There are quite a lot of tutorials about CC Particle World which could give you the basics.