how do I export to make my animation background transp. in AE?!?

hi

I´m a newbie in illusion and don´t know how to create an animation with a transparent background!!!
my problem is quite simple I think?!? I want to use the output animation without any background to position it free and where I want in AE. I´ve got many layers in AE that are already animated and want the particles to flow around and over them. therefore I need your help;)

hope you´ve got time for me left...

greets

Marcel