Before you play your first ranked game, make sure you have:
1. At least 30 casual games played. You need to understand the basics before you play ranked.
2. At least 2 heroes you're comfortable with in your main role. Your main might get banned or picked.
3. At least 1 hero you can play in every other role. You won't always get your main role.
4. A basic understanding of all 5 roles. You need to know what your teammates are supposed to be doing.
5. The right mindset. Ranked is about improving, not about winning every game. You will lose. That's okay.
If you queue for ranked without these, you're going to have a bad time — and so will your teammates.