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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 -Sy 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.