Your day will be filled with both excitement and a subtle fear of the unknown. Both are normal, both are to expected, both make the day that much more fun. I suggest a light meal and hour or so before you jump. Comfortable clothing, in layers, if you are in a cooler climate. Sneakers on your feet.
Arrive earlier than your scheduled time if you have such an appointment. Enjoy the dropzone, talk with other jumpers, ask questions. Skydivers love to talk, especially about skydiving.
As a TI, there are two things more than others that I teach all of my tandem students, they are basic and important. One, is to breathe. You can breathe normally in freefall. The air is free, but self-serve. Two, ARCH. This is the typical freefall position. It is simply when your bellybutton is closer to the ground than you shoulders and knees. Your instructor will work with you regarding both of these.
The main thing is to have fun. Even if you never jump a second time you will never forget it. Get a video, its great showing it off after T-day dinner!! Two words that get you in trouble, ALWAYS and NEVER