Hi,
Firstly, if you code in Swift and/or Objective-C then what you learn for iOS will remain true for OS X. Some will argue that Swift is the language of the future and they may be right. You'll find more resources online for Objective-C though, so learning
may be easier. So find yourself a good guide to Obj-C or Swift - don't worry too much about whether it's aimed at iOS or OS X. Maybe try:
https://www.raywenderlich.com/category/os-x
What
is different between the two is whether you're dealing with Cocoa or Cocoa Touch, which is the UI framework used by OS X and iOS respectively. So don't waste too much time on learning Cocoa Touch if you know for a fact you'll only be doing OS X work, and vice versa.
You might find some of the guides here helpful:
https://www.bignerdranch.com/we-write/
Also, the official docs here:
https://developer.apple.com/library/mac/navigation/
Best of luck!
r.
* Although you
can create apps in all sorts of other languages (Python, AppleScript etc), personally I'd stick with Swift or Obj-C unless there's a really compelling reason not to.