Most PTs will have a consultation session with their clients prior to their first session so that they can understand their current fitness levels, goals, and lifestyle is, so that the PT can build out a tailored plan.
There is no set rule for what a first session looks like, but it may include:
A run through of your new workout programme to make sure you are comfortable with the exercises.
Asking you to perform certain exercises so the PT can check your form and understand your baseline level of fitness.
Taking you through the first workout of your new programme.
You may have your measurements and photo taken.
You will also have the chance to ask any questions, whether that's about nutrition, certain exercises, or tracking your progress.