First you have to download forge and install it http://files.minecraftforge.net/maven/n ... 1.8.9.html
Then download pixelmon and place the pixelmon mod on your %appdata .minecraft mods folder http://pixelmonmod.com/downloads.php
You may also use the Pixelmon launcher which can be found on the downloads page as well
https://www.youtube.com/watch?v=RewcZUpwQcU