Just go to class! Muay Thai will get you into the best shape you have ever been in your life if you attend regularly. People will understand that you are new.
At a class you can expect Thai pad and focus mitt drills, heavy bag work, calisthenics, and in ring drills. It is pretty intense!
The first time I went to a Muay Thai class I was frustrated that I couldn't keep up with everyone else! But, as I continued going to classes, I continued to improve.