« I am running Debian and I sadly need to add non-free software on top of it.
I realize I will give up some of my freedoms by doing so. »

Here is your /etc/apt/sources.list file:

# free software, brought to you by Debian
deb http://http.debian.net/debian/ stable main

# non-free software, here be dragons
deb http://http.non-free.org/debian/ stable contrib non-free