You will need a Reverse Osmosis system to reduce Fluoride in drinking water. The RO membrane is what reduces the fluoride, nitrates, barium, sodium, etc in water.
Here is our most popular RO system. The cartridges are really easy to change.
http://www.waterfilters.net/Good-Water-Warehouse/Reverse-Osmosis/systems/PuROTwist-4000.htm
A regular under sink system does NOT reduce fluoride. The system is less expensive but does NOT have a RO membrane which reduces the dissolved solids. One of our most popular under sink systems is the:
http://www.waterfilters.net/Culligan-US-EZ-4-Under-Sink-Filter-System_p_0-434.html
Thank you,
WaterFilters.NET