How should I train/prepare for starting Muay Thai?

First and foremost, like the others said work extra hard on your cardio. Put the work in now and you wont pay for it later. Also you really want to focus on your balance. Everyone knows you need balance in any martial art but few focus on true development of this skill. Also focus on building leg strength. That was my biggest problem when i first started. The amount of strain on your calf will be a shocker. Jump rope and shadow box as much as possible. I recommend 2 to 3 3min rounds a day at least. Good luck I know you will love the art of muay thai once you settle in.