I normally just wear shorts and a t-shirt, especially in the summer time. There are a lot of small shops that sell trendy t-shirts, some of the trendiest ones right now include Once Upon Apparel, Pixie Lee & Co, and one of my personal favorites, Pretty Little Monograms.
As far as shoes go, Adidas with cloudfoam were a game changer for me, and you can find them at Kohls for pretty cheap.
wear shorts or jeans/leggings depending on when your going. During the months of December, January, February, ect. it gets chilly and quite cold at night. so you may want a sweater. Stay away from the flip flops at the parks because you will be doing lots of walking. I suggest runners or converse (that's what I did and my feet were fine). There are also some water rides so keep that In mind too. You can really wear anything you want to. The parks for the most part just effect your feet. And it rains in Florida close to every day so a rain jacket might not be a bad idea.
Okay, so for foot gear, totally go for strap on sandals if you're going anytime between March and November. Even wearing them in the winter months is bearable. But because of the chance of rain, it's HORRIBLE to wear tennis shoes, even waterproof ones. The water finds a way in. Trust me, I spent two days at Disney in actual pain because of wet shoes.
Clothes wise, I always go t shirts and jean shorts or a short skirt. There's a bunch of cheap Disney tees on Amazon, and you can even find them at Goodwill too.