how should i do, if i would like to enroll a class here...:)

Do you mean you want to take a lesson form one of the teachers in italki?
If so,

1. first, you must purchase credits
2. selsect teacher
3. push the "schedule sessin" bottun
4. choose a course
5. select a day and time when you want to take a lesson
6. write a comment if you have
7. input your passward.
8. wait the answer from the teacher if she/he accept your application

thanks...:D