If you just installed i3 on a laptop and for some reason you can’t tap or the scrolling behavior doesn’t like you, you can update how the touchpad behaves using a xorg config file.
In my case the scrolling behaviour wasn’t the one I like, so I modified the NaturalScrolling property.
## /etc/X11/xorg.conf.d/30-touchpad.conf
Section "InputClass"
Identifier "touchpad"
Driver "libinput"
MatchIsTouchpad "on"
Option "Tapping" "on"
Option "NaturalScrolling" "true"
EndSection
You can see all the available options.