Make sure your /etc/network/interfaces file looks like this:
iface lo inet loopback
iface wlan0 inet dhcp
How do you find your essid and access point (ap)? In my case, I cheated by using the "Network configuration" icon on the top left. If you click on it, it will show the available interfaces, just pick the correct one. Then issue the iwconfig command on the shell, and you should see the essid and ap.
lo no wireless extensions.
eth0 no wireless extensions.
wlan0 IEEE 802.11b+/g+ ESSID:"my-essid" Nickname:"acx v0.3.36"
Mode:Managed Frequency:2.437 GHz Access Point: 00:25:43:DE:AC:2B
Bit Rate:24 Mb/s Tx-Power=15 dBm Sensitivity=1/3
Retry min limit:7 RTS thr:off
Link Quality=31/100 Signal level=3/100 Noise level=0/100
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0
You can also find these parameters by logging into your wireless router's admin interface. You need to wire the computer to the router (or use another computer connected to the router) for this.