By default, if you using LXDE desktop environment, you can brightness/volume up or down using Fn + Key. To make these key work on LXDE you must edit some file.
Open File Manager (PCmanFM), show hidden files.
open .config/openbox/lxde-rc.xml with text editor.
Then add this code:
<keybind key="XF86AudioRaiseVolume">
<action name="Execute">
<command>amixer set Master 5%+ unmute</command>
</action>
</keybind>
<keybind key="XF86AudioLowerVolume">
<action name="Execute">
<command>amixer set Master 5%- unmute</command>
</action>
</keybind>
<keybind key="XF86MonBrightnessDown">
<action name="Execute">
<command>xbacklight -20</command>
<startupnotify>
<enabled>yes</enabled>
<name>Decrease screen brightness</name>
</startupnotify>
</action>
</keybind>
<keybind key="XF86MonBrightnessUp">
<action name="Execute">
<command>xbacklight +20</command>
<startupnotify>
<enabled>yes</enabled>
<name>Increase screen brightness</name>
</startupnotify>
</action>
</keybind>
Before </keyboard> line, then save, logout, and login again, then test it out.
Hope it works. Enjoy.. :)