How to advise a high school student about choosing a university to study psychology?
Psychology can be roughly summarised as the study of human behaviour. To figure out where your interest lies in the field, you have to figure what aspect of human behaviour you want to understand and how you want to understand it.

Do you want to know:


How people solve problems? That’s Cognitive Psychology.
How we can model how people solve problems with a computer? That’s Cognitive Modelling.
etc…


There are a lot of fields and they aren’t mutually exclusive. To get started, you may want to browse the tags of this site or look at the Wikipedia outline of the Psychology topic.

Side note: Wanting to help people isn't the best reason to want to study psychology. There a much more effective ways of reducing suffering in the world. See 80,000 Hours for further information.