Put a few hundred rounds through the pistol you'll be using while concentrating on getting comfortable with its trigger, mag swaps, clearing malfunctions, etc. Work a bit on fundamentals; he'll likely give you significant corrections on them, but a few hundred rounds' worth of bad habits won't be deeply ingrained yet.
At the session, pay attention to things you can't do for yourself: things like which drills he teaches you (and ask him why), what he tells you that you need to work on/correct, and so forth. At the end of the session, be sure to ask him what you work on going forward; he'll likely be able to give you a few more drills and some pointers for ongoing practice.
Lastly, recognize that learning to shoot is a long long process. I've been firing handguns for 8 years or so, and I wouldn't consider myself good.