Installing a gui is easy. All you have to do is pick the one you want then install it. For example, say you want to install openbox:
pacman -S openbox
Of course, since you are new, you will probably want to install something like GNOME:
pacman -S gnome
or Xfce:
pacman -S xfce4 xfce4-goodies
But what you really need to do is spend a good amount of time reading the Arch wiki. It can be a bit hard to understand at first but it will give you more info than anything else. Oh btw, don't forget to install xorg and your appropriate video driver first.