When I first started chess, my mentor asked me to learn one opening that I like and stick to it, study it and master it. But things will not always go the way you think, your opponent might not always start `1. e4`. However, starting `1. e4, 1. d4 , 1. Nf3...` might leads to the same position after a sequence of moves (considering the in between moves to reach that position does not consist of a blunder). 

What is the most common opening position(s) / structure(s) to be in regardless of the starting move? This is because by studying those, I think I can try to shape the board into those positions which I'm comfortable in.