First, install xrandr. If you haven’t done so (and are using Arch Linux or a derived distro):

sudo pacman -S xrandr
xrandr

The output will indicate which monitors are available. To activate the second screen, type the following command:

xrandr --output $SECONDARY_DISPLAY --auto --right-of $PRIMARY_DISPLAY

A new workspace will be automatically instantiated in the second display.

If you want to move containers between displays, add the following lines to your i3 config file:

# move focused workspace between monitors
bindsym $mod+Ctrl+greater move workspace to output right
bindsym $mod+Ctrl+less move workspace to output left