Wifi adapter user manual
From Wiki
Step 1: Insert Wifi adapter into the USB interface of the Raspberry Pi.
Step 2: Start the Raspberry Pi, modify the network configuration file under /etc/network/interfaces. After modification, it is shown as follows:
auto lo iface lo inet loopback iface eth0 inet dhcp allow-hotplug wlan0 auto wlan0 iface wlan0 inet dhcp wpa-ssid "your_wifi_SSID" wpa-psk "your_wifi_password"
Step 3: Save, exit and restart the Raspberry Pi.
Type ip addr on terminal and press Enter, you should see the Raspberry Pi has obtained the IP address by Wifi Adapter.