Here is a short blog about cross platform development and general advice: For intermediate programmers

• If you are working together in a team and just want to make a game together, use a game making tool like unity. This can be also true if you don't have much time during the day.
• If you are more math oriented than programmer
  • Learn object oriented programming and class or object reuse
• Start learning the differences between code libraries on linux, windows, and mac computers
• use c,c++ programming language or java is also acceptable for beginners
• definitely learn opengl for cross platform development
• lookout for differences in multithreading and concurrency API calls and libraries between platforms
• start learning software engineering and get a good handle of modularity
  • start practicing UML diagrams, these will help you identify places in the game engine where differences between operating systems might happen, and how to neatly create different code set variations when needed
• look out for audio libraries, licensing, and codec information on different platforms; windows provides many codecs for free as a cost for purchasing windows, but maybe different on other platforms
  • ALSA on linux
  • xaudio2 on windows

Happy Coding!!

Update: While I still endorse learning opengl for a variety of reasons, including opengl es and webgl, and as well as for legacy considerations,

I'm adding:

definitely learn Vulkan for cross platform development. It's structured similarly to DirectX12, so if you are thinking about learning both, its a great technology to learn.