To pick a role for the season, play every role for a while — you'll learn what you enjoy: pace, responsibility, and playstyle.
You can change roles anytime. But for real improvement, stick to 1–2 roles:
- one main — the role you play most games on;
- one backup — for when matchmaking puts you elsewhere.
Once your roles are set, build a hero pool — three heroes max on your main role. How to do that is in How to Play Better in One Season (the hero pool section).
Season formula: role → pool → repetition. The less you spread yourself, the faster your skill and game sense grow.