Graduate school is like the previous 16 years of school in that you learn things and take classes, at least for the first couple of years. On the other hand it’s also much more like a regular job than the previous schooling. You do work that’s unrelated to class, and you get paid. Maybe not a whole lot of pay, but it keeps a roof over your head and food on the plate. Roughly speaking the paycheck will come from one of two things: teaching assistantships or research assistantships. In the former you are involved in teaching classes/labs and grading the resulting work. In the latter you do research under the direction of your advisor.
As of yesterday I have taught what is probably my last class as an RA. As of next semester I’ll be supported under a research assistantship helping on a new project, which sounds like it’ll be especially interesting even compared to the rest of of amazing work done by the Texas A&M quantum optics and laser physics groups. It’s difficult work requiring initiative and dedication, but I think I’m up to it.
A little over a century ago there was a famous essay written by the title of A Message to Garcia. I happened to discover it just a few months ago, and I think it describes the mindset that’s pretty much ideal in a graduate student as well. It’s an interesting and classic piece; you might enjoy reading it if you have a few minutes.