Because that's a question I'm already sick of getting asked here, do this:

First you need the mission file downloaded, an easy way to get it is to have a friend host it from their personal pc, join it, launch the mission and arma downloads the mission file to %appdata%\Local\Arma 3\MPMissionsCache, from there you can place the mission file to the arma3\mpmissions folder on your dedicated server, then follow the instructions here: https://community.bistudio.com/wiki/Arma_3_Dedicated_Server