I just got a girl’s phone number at school today. What should I say when I text or call her for the first time?

First, you can talk about some topics that interest him. This can be based on her major, and then look at her clothes to see her personality. You can't talk about that aspect at the beginning, because usually In the eyes of girls, that is very abrupt and uncomfortable, and will inadvertently create a feeling of rejection towards you. There should be nothing else, besides being polite, haha, I wish you success