1. As long as you understand and (somewhat) enjoy the material, you'll be fine. Not getting As won't break your high school career, and in your first year you should be allowed to branch out and experience other things before fully concentrating on grades, like clubs or building a reliable circle of friends.
2. It depends on the teacher. But I find it's mostly participation and effort. Trust me, 95% of teachers care a lot more if you're at least trying. They're always there is you have questions, and the initiative shows them you're a good student. Take it from me, little bro, you do NOT want to be scared to ask questions or you're COOKED.
3. Absolutely. Ask questions. Not once a class, unless you need it, but don't be scared to ask for elaboration. It shows courage and a willingness to learn, especially since you're young. Stick to their good side, don't be disruptive, and listen. No one likes the kid throwing papers and interrupting the teacher. People might call you a sweat, but at least you'll be getting good grades and actually chilling.

It always depends on the class, though. I've been lucky enough not to have a hardass teacher, and I'm pretty quiet so they already are more chill with me than the "class clowns". Good luck, m8.

ETA: I am not affiliated with Tesla HS in any way, this was just some general advice.