How to create my own mobile os?

If you are creating the operating system, there won't be an SDK. You'll essentially be developing the SDK for other developers (unless you want to control the OS features entirely by yourself). You may want to find some books on operating system development. My college degree isn't in computer science, but some of my coworkers did create operating systems in school. They were generally pretty simple (at least by end user standards). They basically built a motherboard and wrote an embedded operating system to make a simple game. To make a mobile operating system, are you working with your own hardware or existing devices? You'll probably want to get the manufacturing specifications so you can figure out how to write your device drivers (especially if you plan to support multiple hardware configurations).