Anxiety over subbing?

So I’ve never taught before and am doing subbing as a second job because I want to get out of my comfort zone, I like school, and the flexibility of it is nice. But this morning, I woke up for my first job hours before it began, and had to cancel due to panic and anxiety… (someone picked it up right away so I didn’t leave anybody hanging.) Not a great start. It was literally for high school AP English; couldn’t get any easier. Any tips for how to combat this so I can get used to the classroom/commanding a room better? I have a few other jobs lined up and really want to get comfortable with all grades but am worried about feeling anxious or not knowing what to do in situations. I look very young, too, and want to be respected by the students.