I have a notebook full of various thing regarding my game, but I don't want to get ahead of myself with certain aspects. What part of the game is the most important to flesh out before continuing? Should I start with writing out the entire campaign/story/lore? 

Should I develop some type of art style that I want for my game? 

Should I start coming up with roughly balanced items/units?

I'm new to this and very eager to jump headfirst into whatever it may be, but there is so much to do I don't quite know what is the most important to get done as a framework for the rest. For now I'm going to structure the campaign story, but any advice regarding what is a good order to do things in that wouldn't slow things down is helpful (not that I'm rushing to get it done but backtracking sucks)