I don't really view "choosing a main" as picking out specific qualities you want your main to have, or what tier-rank you want your main to have, or anything even dealing with technical aspects. Really your main should basically come to you. You want to find a character you enjoy playing, who's moveset you like, which has the best learning-curve dependent on you, yourself. You can have someone choose a main for you, but that won't do you any good if you don't actually like that character. I found my main simply by picking my favorite videogame character, and turns out I liked him, so I stuck with it.
Just experiment with a bunch of different characters, and find the one you have the most fun with, and work from there. You don't have to restrict it to 1 or 2 by the way, I know people who have like 10 secondaries, it's ridiculous.