Where are you training?
I would recommend concentrating on the basics for now.
Make sure you're eating enough food, especially protein.
If you're training hard, you're going to need at least 2/2.5 grams of protein per kilogram of bodyweight. If you don't eat enough, you won't be training effectively. What you eat doesn't matter so much, unless you're overweight. You don't see overweight thai boxers for good reason. Drink as little alcohol as possible. If you're smoking cigarettes, stop right now. Increase your water intake when you're not training. Have a snack before training (fruit is ideal) and a meal afterwards.
Make sure you are sleeping enough for your body to recover.
I used to sleep in between training sessions when I was training twice a day! Make sure it's a good, interrupted sleep ie switch your phone off. Go to bed at a reasonable hour. Don't stay up all night shagging your girlfriend. This is as important as taking in enough food.
Run as much as you can.
Running helps you in several ways. It makes your heart stronger and ultimately helps you achieve a lower resting heart rate. It increases your lung capacity. It thickens the muscles in your legs. If you're not used to running then at least try and get a light schedule happening that you can stick to. Start off with 2kms, then when you're not gassing out increase it to 4kms, then 6kms, ideally you want to be putting in at least 10kms every time you're running.
Relax
A common mistake with beginners is to put 100% power into everything. This is a really bad idea. In muay thai there is a lot of muscle memory in every movement. If a hard kick is coming at your ribs you don't have time to think "ahh right, it's a right kick at my ribs that means I have to check with my left leg that means I have to turn my left leg out slightly and raise my knee up to the inside of my elbow and ensure there is no gap between my elbow and my knee etc" NO. Relax and become technically good with the techniques you're learning. After this the speed will come, and then the power. This is a natural progression and doesn't need to be forced. If you're not relaxed, you may be able to strike harder in the short term, but in the long term you won't improve.
Use your head
I don't mean use it to get knocked out. It's important that you understand what you're doing in your own mind. How to do any offensive or defensive technique properly and why you're doing it. When should you attack your opponent? If your opponent moves this way which way do you move? Whens the appropriate time to clinch? How do you work someone into a corner? What do you do once they're there? Your brain is the hardest muscle to train and the sooner you start training it the better
Listen to your body
Common mistake beginners make is they do a bit of muay thai and love it, then they think "awesome, I can train 5 days a week" and pretty soon they are at a high risk of injuring themself or somebody else. If your body is telling you something hurts, or telling you it's tired, then DON'T TRAIN. The first few weeks are the hardest for this but there are injuries you will sustain that will only get better with rest like like blisters on your feet, bruised shins, black eyes etc.. You're not a professional yet, so there is no need to train like one. Step up your training gradually in intensity and frequency and you're less likely to lose training time from injuries in the long term.
Listen to your kru
You might think you're good after a few weeks, but you are never as good as you think you are. I see a lot of questions about "which muay thai book or dvd will turn me into a supreme warrior from home" the answer is NONE OF THEM WILL! You can't see how your technique looks. You can't spot the basic mistakes you are making. Your kru can so listen to everything they are telling you. Be a sponge. If you don't understand something then ask them. If you still don't understand it then ask them to clarify. You're going to look far more silly from getting KO'd in a fight than by asking them to repeat something you don't understand.
Basics basics basics
At first muay thai looks easy. How hard is it to punch someone, there are 100s of fights in pubs and bars every weekend, right? Take time to look at how these people are fighting. Their guard is ****, their footwork is non existant, their breathing wouldn't last them 30 seconds in a ring fight. It's important that you nail these kind of things instead of trying to move onto flying elbows, superman punches or chorakee fad hang. Muay thai is incredibly technical, and if you don't get a good grasp of the basics, you'll never achieve your fighting goals.
Finally, don't forget to have fun
The Thais don't do anything unless it's fun, this includes thai boxing. Make friends with people in your class, keep records on your progress so you can see how far you've come, talk with other people who are into thai boxing online, watch fight dvds and go to local fight nights etc..
If you're not having fun with it you need to rethink the way you are training. Even sparring should be fun and if you're just getting your head smacked in every session it's not so much fun for you so talk with your kru about any training concerns you have.
Sorry if I've rambled a bit (a lot)! Anyway, good luck!