How to build confidence?

I will going into either a Year One or Two class in March as part of my PGCE course. My last placement was cut short because I was ill for two weeks and they said I need to redo the placement. This was back in December and I haven't been in the classroom since then. One of the biggest criticisms I had in my observations was that I lacked confidence and presence in the classroom. I know part of that was being placed in Year 6 and teaching maths (neither of which I was comfortable with). So I was wondering does anyone have any tips on how to build confidence in the classroom? Building on that: how do you have authority in terms of talking to teaching assistants?

TLDR: How do you build confidence in the classroom?