You need to do your homework ahead of time for this class. Not every instructional approach will work equally well for every dog. You know your dog's personality better than anyone; keep it in mind as you look at different obedience instructors. A nervous or eager-to-please dog probably won't do as well with an instructor who acts like a drill sergeant, but an outgoing, strong-willed dog will probably ignore a gentler approach. A good instructor will try different methods for different dogs.
Before you sign up for a class, ask if you can sit in and watch the instructor at work with another class. If that's not possible, talk to the instructor to find out his or her approach to obedience, and don't be afraid to ask a lot of questions. Some instructors may have training beliefs you disagree with. Go with your gut instinct. If you don't like the way the instructor treats dogs (keeping in mind that a good instructor has to be firm), or if you just don't think you can get along with the instructor's personality, find a different class.
There are other things to consider. If you know your dog goes crazy with excitement when a lot of dogs or people are around, try to find a class with a small number of participants. Ask ahead of time what the maximum class size is. If you have a particularly small or large dog, see if you can track down a class for dogs of similar size; it may make your dog feel more comfortable in class to be around other dogs of about the same stature.
If your dog tends to be aggressive with other dogs, you will need a very structured approach, and you should consider taking individual lessons before signing up for a class. You'll also want to make sure the instructor you pick has lots of experience working with difficult dogs. Your vet is a great person to ask for ideas in such a situation.
A dog who is aggressive with people is a different problem. Your vet can recommend a good behaviorist who will work with you and your dog individually. The behaviorist will also let you know when your dog is ready for an obedience class and can help you find the right one.