i am going to Larton for the jumping, but my pony has never been in an indoor. Shocked
has anyone got any tips to keep her calm in there?

I'd just take is slow with her. When I was leasing a green horse, he didn't like indoors ethier so what we did was just free lunged him in there & lead him around a bit, then he was fine

Unless you have information to confirm, I wouldn't automatically assume there is going to be a problem.
Does he live in a stable/barn, Yes- then he does indoors.
Most of the "problems" horses have are owner/rider problems.
Of coarse there are those horses who don't like indoors. You need to stay calm and reassuring, be very aware of your breathing. If you are truelly scared/nervous don't ride, your horse will be bad. If you need to have him rode, find a catch rider. But if you can contain your nervousness, just ride, the same as any other day any other ride. If you don't create a big deal, there won't be one.