The main Raspberry Pi website has a good section on how to configure WiFi networking

Raspberry Pi WiFi Networking Section

We'll step through this here as an example using Raspbian Wheezy, this process is the same on Jessie.

When selecting a mPCIe WiFi card remember that the Raspberry Pi ARM chip does not have a PCIe interface so only cards with a USB host-bus interface are compatible.

Log into the unit via the console or as in the example below via the wired LAN connection and check that the wifi dongle has been detected :

Notice that the WiFi has not yet been configured so the device wlan0 has no IP information

We can now do a scan to see which WiFi services are available

root@raspberrypi:~# iwlist wlan0 scan
wlan0     Scan completed :

          Cell 05 - Address: B0:48:7A:EB:5A:4C
                    Channel:6
                    Frequency:2.437 GHz (Channel 6)
                    Quality=70/70  Signal level=-33 dBm
                    Encryption key:on
                    ESSID:"testwifi"
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
                              9 Mb/s; 12 Mb/s; 18 Mb/s
                    Bit Rates:24 Mb/s; 36 Mb/s; 48 Mb/s; 54 Mb/s
                    Mode:Master
                    Extra:tsf=00000008b028ce28
                    Extra: Last beacon: 10ms ago
                    IE: Unknown: 000664642D777274
                    IE: Unknown: 010882848B960C121824
                    IE: Unknown: 030106
                    IE: Unknown: 2A0104
                    IE: Unknown: 32043048606C
                    IE: IEEE 802.11i/WPA2 Version 1
                        Group Cipher : CCMP
                        Pairwise Ciphers (1) : CCMP
                        Authentication Suites (1) : PSK
                    IE: Unknown: 2D1A4E101BFFFF000000000000000000000100000000000000000000
                    IE: Unknown: 3D1606050600000000000000000000000000000000000000
                    IE: Unknown: DD180050F2020101000003A4000027A4000042435E0062322F00

                    

The above shows the target channel testwifi has been detected AOK so we can now proceed to the configuration section

The file below has all the configuration information we need to alter : 

/etc/wpa_supplicant/wpa_supplicant.conf

Alter the two lines highlighted above with the ESSID name (testwifi) and the password for the network

Once saved it's not uncommon to have to restart the interface and allow about 5-10 seconds for the DHCP process to complete

More detailed configuration for the WiFi networking setup, including DHCP/static IP information, is contained in the file below :

/etc/network/interfaces

Note that you should not need to edit this unless you need a custom configuration

If you run into problems with starting the wlan0 interface like the below :

root@raspberrypi:~# ifdown wlan0
root@raspberrypi:~# ifup wlan0
wpa_supplicant: /sbin/wpa_supplicant daemon failed to start
run-parts: /etc/network/if-pre-up.d/wpasupplicant exited with return code 1
Failed to connect to wpa_supplicant - wpa_ctrl_open: No such file or directory
wpa_supplicant: /sbin/wpa_cli daemon failed to start
run-parts: /etc/network/if-up.d/wpasupplicant exited with return code 1

This can usually be resolved by running wpa_passphrase to generate alternate configuration information for use in /etc/wpa_supplicant/wpa_supplicant.conf

root@raspberrypi:~# wpa_passphrase testwifi wifipassword
network={
        ssid="testwifi"
        #psk="password"
        psk=c59eb9956f67f1adfddddddd3dd330f0384c5a5d391cecb1abcb1271c8f095c3
}

In the above example testwifi is the SSID and wifipassword is the password.

Alter the wpa_supplicant config file with this information for the network section and then reboot.

If you are using a mPCIe WiFi card you can disable/enable the card using

# echo 1 >/dev/pcie-disable

# echo 0 >/dev/pcie-disable