You can look in the events section of this website and they will list all regional MCA shows/events and it is a good idea to keep checking as events are added at different times. It was very helpful to me to go to the judging section of this website and download a copy of the classes to determine what class I would be in and then download the judging rules for that class to see where the most points are and concentrate on those areas. The Grand National MCA show is in the midwest in 2012, at Mustang Oklahoma, but I would suggest you try a regional show or two before showing at the Grand National.
If you have access to a local MCA club, I would suggest you join it and try to find a gold card judge to talk to for some advice. When you do enter a regional show be sure to request a copy of your judging sheet so you will know where points were deducted and get an idea of what to work on for the next show. The best advise is to clean, clean, and then clean some more.
Good luck and have fun with the experience.