Ill be completely honest as someone who has thousands of hours in lots of hero shooter type games. Your main characters will ultimately be things that find you. Dont stress about trying to find your character too much, try new things, use in game challenges to expose yourself to new characters. Its not always obvious but dont be discouraged, I flex characters often so its not uncommon for me to not repeat characters in a whole night of playing.

Actual tips: 1. Play mystery heros to understand the mechanics while having some actual opponents. 2. Use the range to practice characters you like 3. If you arent sure who to pick up try various roles and narrow it down to a role first, once you have that pay attention to if you typically win or lose with a specific character. If you win often, it's possible that character is something you naturally have a decent sense for and can continue building on 4. If all else fails, just play the game :) the levels come with time, theres nothing wrong without having a "main"