Could I be mentaly ill ?

That doesn’t really sound like a mental illness to me, It just sounds like you might need to try and be more confident about going to act in front of the class, pre-performance nerves and what not :). Just try to keep as calm as you can before you go to drama lessons etc and maybe talk it over with the teacher of the lesson, perhaps they could make it so you didn’t have to act as much - until you felt comfortable with it.

Anyway I don’t think that you’re mentally ill from what you say so you shouldn’t worry about it too much, If you’re really worried about it perhaps ask a doctor for some help maybe they can get you in touch with someone that could help :)

The reason that it is so hard is because you are introducing your body to somthing new. Instead of focusing on one area of your body to a specific thing you are introducing the entire body. For the first few weeks of it keep a small smooth stone in your hand and while acting focus your energy on rubbing your thumb against it. After the first couple of weeks your body will naturally adjust to the new activity and you should no longer need the stone. That will help with the shaking knees, and give you some comfort from the jitters.
There are a lot of people that have this problem, there is nothing to worry about.