1. Start today. Many people feel they need to get in shape, or lose weight before they come to class. Going to an academy sooner rather then later will help you with your goals earlier. Lots of people are in poor shape when they start. No one will even care or notice.
2. Cleanliness is Godliness. Make sure you cut your fingernails/toenails as these turn to deadly daggers when left unattended. Shower sometime before and soon as possible after class. Wash your gear (including gi and belt) after EVERY class. Some people think that washing your belt "washes off the learning" you have done. What it more accurately does is wash off the staph, ringworm, and other bacteria that festers on your belt.
3. Buy a decent gi. You will likely use your gi hundreds of times, so don't cheap out too much. You don't have to buy the latest camo signature lucky model either. If you get too cheap of a gi, it won't last. I would recommend Koral MKM gis. You can get a plain white one for around $140, but it is the best fit out there IMO and has great durability. If you would rather spend a little less, check out Padilla & Sons. I have heard they are a great bang for the buck. Atama, Keiko Raca, Vulkan, and many other brands make good gis. Ask the instructor at the school for their opinion on brands and sizing.
4. Consider watching a class first. This will give you an idea of how the class is run so you know what to expect. It can be intimidating walking into an unfamiliar environment, but trust me, once you start you will be kicking yourself you didn't begin sooner.
5. Don't be afraid to ask questions. Ask the instructors any questions you might have, or share any concerns you might have.
6. If you are allowed to roll during or after class, try and practice the moves you learned in class. I know you may have seen a cool flying armbar on youtube, but you will progress much faster if you focus your learning on the technique and strategies being taught.
7. Roll with as many better people as possible. Don't be afraid to ask questions. If they pass your guard, or hit you with a submission..ask them what they did and how they did it. Ask them if they might show you how they did it sometimes.
8. Don't expect to fit in right away. You will probably notice right away that everyone is very friendly with each other and having a great time. Feel free to join in the fun! Realize though, that these training partners have become close over months and years. They see many people come and go and quit all the time. Once you have proven you are going to come to class and not quit, people will warm up even more to you.
9. If you have concerns, don't internalize them; talk to an instructor. They are there to make your training as fun and productive as possible. They have heard it all before, so be open if you something is bothering you.
10. Come to class early. It takes a few minutes to put on your gear. This is also a good time to get some extra pre class stretching/rolling in. You can also get a little more personal time with some of the students to start feeling more comfortable.