Creating OS

Hi guys I am new here. I want to ask all of you about creating an os (Any kind of Android or PC) what do i have to do to create an os for my personal use or to sell it to someone (Making it private) .Do i have to create OS only or hardware's too . I am free next month so i am thing of this so please tell me about legality or illegality of this. Sorry if it sounds stupid

Thanks

__NONAME