This instructable shows you how to connect Raspi to Internet with WiFi adaptor or WiPi.

Step 1: What Do We Need?

Raspi
Wifi adaptor

Step 2: Preparing to Edit Interfaces

Open LXTerminal and write:
sudo su
apt-get install mc
(wait for question) y
cd ..
cd ..
cd ./etc
cd ./network
mc
Go with cursor to interfaces and press F4.

Step 3: Editing Interfaces

Now you are editing the interfaces file.
Rewrite it to this:

auto lo

iface lo inet loopback
iface eth0 inet dhcp

auto wlan0
iface wlan0 inet dhcp
wpa-ssid (name of Wifi)

wpa-psk (password of Wifi)

Finish!!!
Now press ctrl+x and than y than press enter write exit and reboot.
You can try your wifi conection!