You shouldn't need to be hugely prepared really, like most martial arts, Muay Thai is a great and often fun way of getting in shape.
When I first started I was badly out of shape and couldn't keep up with the rest of the class, but I soon found I was able to keep up with the best of them.
A Thai master, Panya Kraitus, taught us in a seminar before and he insisted on cardio cardio cardio. His favorite exercise was skipping, so I guess that's as good a place to start as any if you want to hit the ground running.
But any good instructor won't expect Bruce lee in terms of fitness and class from a beginner, just ween yourself into the methods and the standards and before long you'll be able to flying knee, spinning elbow, right hook and low roundhouse with the best of them!