You should use netctl.  It is the Arch Linux built in network management system.  If it isn't already installed (it should be though), run `pacman -Syu netctl` then look in `/etc/netctl`.  There are example configuration files in `/etc/netctl/examples`.  To use an example, just copy it to `/etc/netctl` and edit it to fit your set up.  So for your case, run `cp /etc/netctl/example/wireless-wpa /etc/netctl/somedescriptivename` then edit `/etc/netctl/somedescriptivename` to fit your set up.  
To enable auto configuration of your wireless networks with systemd, make sure the `wpa_actiond` package is installed and enable the service: `systemctl enable netctl-auto@wlan0.service`

For more information please refer to [the Netctl wiki entry][1].


  [1]: https://wiki.archlinux.org/index.php/Netctl