that's so sweet! I love it!
I echo what others have said, wear something relatively easy and quick to get on and off.
The worst part of changing is getting all those dang layers off and on again.
Shoes you don't have to lace are a godsend.
But at the same time you want something that has a heel or platform of a similar height to what you usually wear, it means you can see how trousers and skirts will fall with shoes on and whether anything needs to be taken up. (Oh look at me with my female socialisation lol)
When I had long hair i'd usually either tie it back or have it loose and unstyled because pulling things over your head repeatedly is gonna mess up any style you're trying to rock.
Also, you'll likely be on your feet and walking for AGES if this is a standard girly shop. So whatever shoes you do wear, make sure they're fairly comfortable, same with clothing.
I know from bitter experience shopping with my mother and sister how a "quick shopping trip" can rapidly become 4 hours of looking at dresses hahah.
Have fun though!