Her outfit it a pale pink, peachy sort of colour (just go with pale pink, it's easier), and her whole dress is covered in embellishments.
She is supposed to be tall and majestic and beautiful, to show that she is good, which is why you should opt for heels and a smile with this costume.
You will need;
-pale pink tutu (bigger is most definitely better, as Glinda's original dress was floor length and extremely wide, wide enough that it covered Dorothy's legs while the muchkins switched her shoes to ruby slippers)
-pale pink body suit (i could only find a white one, or if you're stuck, just a long sleeved pale pink top)
-pale pink leggings (you could probably go without, but it's probably going to be cold, and you never actually see glinda's legs)
-pale pink or silver heels (you never actually see her shoes, but as she is tall, you should wear heels, and pale pink or silver will go best with this costume)
-silver crown (basically a tall silver pointed hat, which you could probably get for like less than $5 in a costume store, it isn't actually a crown that Glinda wears)
-silver wand (it should be a long wand, not a shot one, and with a big star on the end, and same with the hat, you coul probably get this for less than $5 in a costume store)
For your hair, curling it would work best, and leave it down, and pulled to the front.
Soft, natural looking make up with peachy cheeks and coral lips would go best with this.
Also, if you're like some sort of scientific genious, you could make a bubble and float along in it, or if you have one of those big exercise bubbles that you can wlak on water in, it would be so awesome with this costume!