It’s always best to arrive at the airfield a little earlier than your lesson is planned. Sit down, have a cup of tea and soak in the atmosphere, there’s always a lot going on at an airfield that will get you a better feel as to what is coming up. You’ll also nearly always find that there will be others around who will be more than happy to talk planes with you, or give advice and experience.
There will most likely be some paperwork to fill in when you get there too, so make sure you give yourself plenty of time to deal with that.
Normally the flying school will lend you a headset at this point. Aeroplanes are quite noisy environments and a headset is required to communicate between you and your instructor, as well as protect your hearing.