1. Open a terminal, type the following command to view the connected usb ports:


linaro@linaro-alip:~$ lsusb
Bus 001 Device 003: ID 1a40:0101 Terminus Technology Inc. 4-Port HUB
Bus 003 Device 002: ID 0bda:8176 Realtek Semiconductor Corp. RTL8188CUS 802.11n WLAN
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 010: ID 05ac:0304 Apple, Inc. Optical USB Mouse [Mitsumi]
Bus 001 Device 005: ID 046a:0023 Cherry GmbH CyMotion Master Linux Keyboard


2. To scan WiFi access points:


linaro@linaro-alip:~$ iwlist wlan0 scan
wlan0     Scan completed :
          Cell 01 - Address: E0:91:F5:83:54:3C
                    ESSID:"benzaku"
                    Protocol:IEEE 802.11bgn
                    Mode:Master
                    Frequency:2.412 GHz (Channel 1)
                    Encryption key:on
                    Bit Rates:72 Mb/s
                    Extra:rsn_ie=30140100000fac040100000fac040100000fac020c00
                    IE: IEEE 802.11i/WPA2 Version 1
                        Group Cipher : CCMP
                        Pairwise Ciphers (1) : CCMP
                        Authentication Suites (1) : PSK
                    IE: Unknown: DD0E0050F204104A0001101044000102
                    Quality=100/100  Signal level=-45 dBm


3. Edit /etc/network/interfaces


linaro@linaro-alip:~$ sudo nano /etc/network/interfaces


# the /etc/network/interfaces file: 
  GNU nano 2.2.6         File: /etc/network/interfaces                          


# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback


auto eth0
iface eth0 inet dhcp


#append the following lines
auto wlan0
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf


4. Edit /etc/wpa_supplicant/wpa_supplicant.conf 


linaro@linaro-alip:~$ sudo nano /etc/wpa_supplicant/wpa_supplicant.conf


Add the following:
network={
ssid="your_ssid"
scan_ssid=1
proto=RSN
key_mgmt=WPA-PSK
pairwise=CCMP
group=CCMP
psk="your_wifi_password"
}


For reference 
ssid:   your wifi name
scan_ssid:   value of 1 means broadcast and value of 2 means hidden (suggest you enter value of 1)
psk:   your wifi password
proto:   your choice of RSN or WPA. RSN is WP2 and WPA is WPA1. (most config is RSN)
key_mgmt:   your choice of WPA-PSK or WPA-EAP (pre-shared or enterprise respectively)
pairwise:   your choice of CCMP or TKIP ( WPA2 or WPA1 respectively)
auth_alg:   OPEN option is required for WPA and WPA2 (other option, SHARED & LEAP)


Then run 
# /etc/init.d/networking restart
Or
# ifup wlan0

# dhclient wlan0
If this doesn’t a reboot of the system may help. 


(Optional)
script for auto-start wifi connection
/etc/init.d/wifi-connect
#!/bin/sh
#wifi-connect auto-start

case $1 in
start)
 wpa_supplicant -B -iwlan0 -c/etc/wpa_supplicant/wpa_supplicant.conf -Dwext -s
 wpa_cli -B -p/var/lib/run/wpa_supplicant -s
;;
stop)

;;
restart)

;;
*)
echo 'Usage:wifi-connect start|stop|restart'
;;
esac
exit 0

# chmod +x /etc/init.d/wifi-connect
# update-rc.d wifi-connect defaults
# update-rc.d -f wifi-connect remove