Sometimes when you look for a package in https://archlinux.org/packages and try to install, it won't work. This is because you haven't yet added the ArchLinux Repository that contains the package you are trying to install.
Open configuration file "/etc/pacman.conf" using editor "nano":
sudo nano /etc/pacman.conf
add '#' from whatever repo you want to enable
remove '#' from whatever repo you want to disable
eg, if you want to add 'extra' repository, then make sure you have these lines (without a '#'):
[extra]
Include = etc/pacman.d/mirrorlist
or alternatively, to disable 'extra' repo, just add the comment sign '#' to these same lines, ie:
#[extra]
#Include = etc/pacman.d/mirrorlist
save file by clicking CTRL+X , then 'Y' . Now you should have the desired repos available according to how you edited the files in step 2.