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