Been to a few psychiatrists and work with a lot as a clinician. For a first meeting you should expect to give a lot of history including familial history, treatment history, substance use history, medication history, diagnosis history, trauma history, developmental history, etc. Skip any that aren’t relevant to you and if you can’t give tons of info that’s okay (eg if you don’t have tons of info on your early development just give what you can, you can skip substance use if you’ve never had issues with it, etc). description of presenting problems including what you’re coming in for but also cooccurring physical issues that may be relevant (eg chronic ailments like diabetes/gerd/migraines) as well as sleep quality.
You may not touch on all of this, it may include other stuff, depends on the doctor. Each practitioner has their own personal approach but those are the broad strokes. Assuming you’re in the USA remember your rights as a consumer: you have the right to know why you are being prescribed a medication, you have the right to know it’s side effects, and you have the right to know alternatives to it. But at the same time remember side effects aren’t always experienced and for some people being aware of them makes them more likely to occur (I am one of those people fwiw)