When you create user on archlinux, by default your user have no super user access, and if you fresh install sudo still not installed by default. To enable sudo on user, this what i did.
1. Install sudo & gksu
By default you can use su only, but for some apps, for user can be able run using sudo only.
gksu will be needed later when you install some desktop environment. Ex: running root mode on PCmanfm.
#pacman -S gksu sudo
find line like this below, then add below root
If done, CTRL X to save it, then try to sudo
enter your user password
If you want to run apps in super user mode. For this example, leafpad.
Done and enjoy super user mode on your user. :p