Hii!
Please mention the Board of education you are from, the subject stream you are Studying or want to pursue in future. You question ka quite unclear. Mention if you are askig with regards to any entrance exam.
Whole syllabus of class 11th is important as, what you learn in class 11th, you apply in class 12th. So, there's nothing you can pay less attention to.
Hope you understand.
Good luck!