WiFi Drivers

Firmware

Ra-link

sudo apt-get update
sudo apt-get install firmware-ralink

Realtek

sudo apt-get update
sudo apt-get install firmware-realtek

or something like this, but this is included in firmware-realtek package

sudo wget https://github.com/lwfinger/rtl8188eu/raw/c83976d1dfb4793893158461430261562b3a5bf0/rtl8188eufw.bin  -O /lib/firmware/rtlwifi/rtl8188eufw.bin

Atheros

sudo apt-get update
sudo apt-get install firmware-atheros

Drivers

Install kernel headers

The kernel header files are needed to compile the drivers.

 sudo apt update
 sudo apt install raspberrypi-kernel-headers

Realtek RTL8812au

 cd /home/downloads/WiFi
 git clone https://github.com/aircrack-ng/rtl8812au.git

Realtek RTL8188eu

 cd /home/downloads/WiFi
 sudo git clone https://github.com/lwfinger/rtl8188eu.git
 cd rtl8188eu
 make
 sudo make  install

Realtek RTL8192eu

 cd /home/downloads/WiFi
 sudo git clone https://github.com/lord2y/rtl8192eu-arm-linux-driver.git
 cd rtl8192eu-arm-linux-driver
 make
 sudo make install

References

https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=44044

Detailed instructions

uname -a
sudo apt-get update
sudo apt-get dist-upgrade
uname -a