Wireless networking on Linux can sometimes be quite hard if you don't know how to do it. Since you will start in a CLI interface it will be slightly harder to do. It is assumed that you haven't set up a user yet.
Unencrypted Networks
Unencrypted networks are the most easy to connect to. First run iwconfig and look out for some output. Next to the one with the chunk of information, there should be something like "wlan0" or "eth1". If there isn't, you haven't set up yet. Have a look at the Arch Linux wiki. Remember this, as you will need it later as "[network-interface]". Now you will need to connect to your network. If you don't know what your ESSID or SSID is (basically the name of the wireless network), run iwlist scanning and look for your wireless network. Run iwconfig [network-interface] essid [your-essid/ssid] now run dhcpcd [network-interface] If it outputs nothing, your network is now up! Try running something like ping google.com
WEP (Wired Equivelent Privacy) Encrypted Networks
WEP Encryption
It is not recommended to use WEP encryption because it is so insecure. It can be hacked in a matter of minutes! Please use WPA if you can. WEP is not much harder to set up. First of all run iwconfig [network-interface] essid [your-essid] key [your-wep-key] and then run dhcpcd [network-interface] You should then be able to use the internet!
WPA (Wi-Fi Protected Access)
WPA will be a bit harder to set up. An easier way is to use the new netcfg scripts. You will need internet access for this to work! First install netcfg pacman -S netcfg Open /etc/network.d/wireless as root. Put in
CONNECTION="wireless" INTERFACE=wlan0 SCAN="yes" SECURITY="wpa" ESSID="mynetwork" KEY="SomePasskey" IP="dhcp" TIMEOUT=20
And edit the information to your network. Now run netcfg wireless If it runs successfully, your internet connection should now work! Info
You've already done half of what you need to do next 
Getting it to connect at boot
This is the easy bit. See the following page for more information: http://wiki.archlinux.org/index.php/Network_Profiles