I hate interviews sooo much. I remember my first interview being a bit awkward. It helps to go over a list of potential questions they could ask and start to formulate/write down your answers so it's harder to get tripped up by one of them.
I hope it all goes well for you At least it's not some sort of high pressure/ high level anxiety interview like going for a new job. At least you know what you're talking about with the whole volunteering stuff (otherwise you wouldn't be volunteering) so this in itself should give you more confidence