Pretty much. The correct spelling is "until" and the second "I" should be a capital. The "the" before "class" is optional.
Student or Learner
Would it be right to say?
I am not used to sitting in the class untill evening, so i get tired very soon.
You get tired easily or get tired early. "Soon" isn't the right word.
Can you teach where to use "the" and where not?
Note that the use of articles is, in my opinion, the most difficult part of learning English.
Don't be upset if you don't understand immediately.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
'I am not used to sitting in the class until evening, so I get tired.'
We don't need any other word(s) after "tired"; they are all superfluous. If the person has been there since morning or even since the afternoon, the tiredness doesn't need to be further commented on.