Flight controllers are the brains of your UAV and are often the most difficult part to fully understand. Thanks to poor documentation and components which are easily broken this step in the drone process is the biggest crapshoot. Either you get lucky and everything works or you spend weeks troubleshooting before you give up and purchase another flight controller and hopefully do not make the same mistake.
The below searchable list cover some of the best flight controllers currently available on Amazon. I would recommend the Lumenier Micro LUX v2 as it comes with the best "documentation" and a google search should result in guides and youtube videos on how to set it up.