Add a “non-free” component to /etc/apt/sources.list, for example:
Debian 8 “Jessie”
deb http://httpredir.debian.org/debian/ jessie main contrib non-free
Update the list of available packages and install the firmware-realtek package:
apt-get update && apt-get install firmware-realtek
apt-get update
apt-get install network-manager-gnome
reboot
