The first thing to do is start looking at the environment you are going to run in, and the compilers or cross compilers that are available. And then you can start looking at how you are going to use them, given that many basic library functions (such as "get a character from the user" and "print this string") use operating system functions to do it...
This is not a simple task: the chances are that you will want to use a combination of languages: assembler (for the speed and compactness), C for the higher level readability without losing too much speed, and probably C++ to get the higher level abstractions (that will need the most modifications over the life of the OS) that aid readability and maintainability.
Languages you really can't use include Javascript, VB, and C#!
But I don't really think you understand how much work this involves. There is a good book which will be well worth reading:
http://www.amazon.com/Developing-32-Bit-Operating-System-Cd-Rom/dp/0672306557[
^] - but it's a bit basic these days, and it won't get you running with a Windows 10 clone by Christmas! Maybe a DOS clone, if you really put the time and effort in...