The trouble with Muay Thai is you have to understand that knowing the techniques is only about 20% of Muay Thai. The biggest part is knowing what to do in a match. For example you never switch kick an opponent when they are coming towards you or throw a knee if you are moving backward in the clinch. Its these things that are more important.
Another thing is without a trained instructor you could learn a technique and incorrect way or get lazy with your technique, for example letting your hands drop or double punching.
You should focus on getting your cardio up to a Muay Thai level. Many people come to Muay Thai thinking they are fit but just cannot keep up with what is required for a fight.
Try doing this one, it sounds easy but its hard when your just starting.
30 seconds burpies (full press up burpies)
30 seconds star jumps (go all the way down and all the way up)
30 seconds sprint stops (your back should stay flat and your knees should touch your shoulders
repeat 3 times without a break, if you need to throw up do so. BUT DO NOT STOP!
A big part of Muay Thai is the mentality of never stopping no matter how hard it gets. So when you work out run yourself into the ground and don't rest till the end of your workout.
Pain is weakness leaving the body
The more you sweat in the gym the less you bleed in battle