To install yaourt

yaourt: a pacman frontend


git clone https://aur.archlinux.org/package-query.git
cd package-query
makepkg -si
cd ..
git clone https://aur.archlinux.org/yaourt.git
cd yaourt
makepkg -si
cd ..

or add te following to /etc/pacman.conf

[archlinuxfr]
SigLevel = Never
Server = http://repo.archlinux.fr/$arch

Use with

pacman -Su
pacman -S yaourt