In order to start playing Pixelmon you need Forge and the jar file for Pixelmon.
The version of Forge and Pixelmon you need depends on your version Minecraft, so I suggest you look at this updated table:
https://minecraftnotes.com/download-pixelmon-mod/
The above link contains all the download files for every single version of Minecraft that Pixelmon currently works with.
Note that Pixelmon does not work with version 1.11 or 1.12 just yet.