Hey guys,
So I want to make this walk animation with my lego character but i need to integrate the clip into another movie ( i mean with a new layer) so does anybody know how I can get my background transparent? or how i put it on the other movie so that the background disappears? can i somehow import the blender-file into after effects?
Change the RGB output to RGBA in Render panel. Then render to PNG image sequence and import to video editor app
Well thx, but which video editor app fr instance? what do I do there? export? 
could be after effects for example, jakasha also, its free. there are a couple, what you need is a compositing software, that allows you to stack layer of videos/image sequences on top of another, much like image editing software like photoshop or gimp
you can do that in the video editor of blender.