The best way is to start working out before you sign up for classes. Muay Thai is a physically demanding martial art. I would suggest that you start building up your stamina before you sign up for Muay Thai gym.
This way once you start a regular training you will be able to focus on technical aspects of Muay Thai. You could start sparring with more advanced practitioners instead of spending valuable time in the gym to build up your stamina.
Start running. Start doing crunches to toughen your core. Do pull-ups and push-ups to make your arms stronger.
Do not waste a valuable time in the Muay Thai gym trying to build up you stamina. Improve your cardio before you start paying for the class.
You don't need a lot of expensive gear to start your training. All you need is to get is: a pair of quality gloves, hand wraps, mouth guard and obviously a pair of shorts and a vest. The rest of the equipment you will be provided by a [good] gym.
Once you get into Muay Thai, you will probably build your own small workout place with a heavy bag hanging in the corner of your room. Muay Thai can be addictive.
First and foremost: Muay Thai will keep you in a great shape. The level of fitness that Thai boxing promotes and demands is unmatched.
The cornerstone of a Muay Thai training is a bag work/pad work and a sparring - both are very physically demanding activities. Intense Thai boxing training holds a ring-ready level of fitness for a goal. Muay Thai is also a great tool for a self-defense. It keeps you physically and mentally tough.
Muay Thai, also known as an art of 8 limbs, is known for its diverse striking. There are plenty of tools that an experienced fighter can use to control the fight. Lets take a look at basic combinations: