Given your choices today there's absolutely no reason to "make" an OS. Learn a language really well and get really comfortable with Windows or a variation of Linux. C or C++ is great to learn.
Edit for clarification - my point is that it's both unrealistic and borderline insanity to attempt to learn a language for the first time by making an Operating System. At minimum a really good background in C is a requirement, otherwise start making applications and leverage the power of existing OS's before attempting to roll your own.
