Feeling tired after Tai Chi is actually quite common. You might be lacking proper sleep, lacking a proper diet that gives you the energy you need, and lacking proper hydration. Perhaps the workout is more intensive than you are used to.
On your first Tai Chi class you should receive a formal introduction to Tai Chi followed by basic guided warm ups or stretches. You should enjoy a place of quiet, and together with a group of people (possibly also on their first Tai Chi class) learn the first moves by repeating after your instructor.