Check the output of
lsusb and make sure that the device is being detected. Then, check
ip a and see if it shows the device in the list. If not then you'll have to use the information given about the device from
lsusb or if you already know the device's make and model then you can just use that. Then do some research to find out what modules the device requires. Next, check
pacman and the AUR for the modules that your wireless interface requires.
Once installed, you can use
modprobe to load the module and then recheck
ip a to confirm that the device is now being recognized by the system. Then you can use either netcfg or netctl to setup your network network configuration accordingly. Both network managers have great examples and documentation that will help you along the way.
netctl is in the [testing] repository at the moment. You will need to make sure that this is uncommented in
/etc/pacman.conf in order to activate the repository. In the future
netctl will be replacing
netcfg. Best of luck.