First install these packages for brightness and volume:
sudo apt-get update; sudo apt-get install xbacklight alsa-utils pulseaudio
Then from I3 FAQ:
Add these lines to ~/.config/i3/config
# Pulse Audio controls
bindsym XF86AudioRaiseVolume exec --no-startup-id pactl -- set-sink-volume 0 +5% #increase sound volume
bindsym XF86AudioLowerVolume exec --no-startup-id pactl -- set-sink-volume 0 -5% #decrease sound volume
bindsym XF86AudioMute exec --no-startup-id pactl set-sink-mute 0 toggle # mute sound
# Sreen brightness controls
bindsym XF86MonBrightnessUp exec xbacklight -inc 20 # increase screen brightness
bindsym XF86MonBrightnessDown exec xbacklight -dec 20 # decrease screen brightness
To change the $mod key:
Open ~/.config/i3/config.
Look for the line:
set $mod
Change that to:
set $mod Mod4
Finally, run i3-msg reload to reload the configuration file.
Adapted from: i3 FAQ