Non-Free Firmware for Debian
After installing, you may need to install non-free firmware to get the drivers required by your hardware: e.g. to enable wireless connection.
First, allow apt to download packages from contrib and non-free sources:
apt-add-repository contrib
apt-add-repository non-free
Add some common firmware:
apt install firmware-linux-nonfree
apt install firmware-misc-nonfree
# if you have Realtek hardware in your machine
apt install firmware-realtek
For the specific wifi drivers you need, read the Debian docs online. For example, for Intel cards you will need iwlwifi:
apt update && apt install firmware-iwlwifi
# reload kernel module
modprobe -r iwlwifi ; modprobe iwlwifi
For nvidia GPUs, use nvidia-detect to find out the appropriate drivers, e.g.:
apt install nvidia-driver
Check if the nvidia modules are loaded with lsmod | grep nvidia. If not, verify that you had already disabled Secure/Fast Boot in the BIOS/UEFI settings.