i am have done my first jee exam on 2010.i hav got 178 marks.i don't think i could get a good branch with that marks.so i think to repeat and write jee in 2011.can u give me rough ideas to crack iit?

Smart preparation is very important when the time is less, therefore, analyse your strengths and weaknesses and work on them proportionately. For example: if you are good in algebra and need to work on your calculus then devote more time to calculus.
As your Board exams will start way before the entrance test, make sure that you are able to devote enough time for the preparation of JEE Mains in December and January so that you have enough time to prepare for your Class 12 board exams.
Give at least 15 Mock Tests:

With roughly three months left for JEE Mains 2020, make sure that you give at least 15 mock tests in this period of time.
Devote each Sunday for the mock test, score analysis and performance analysis. This will help you to be more familiar with the exam.
If you want to evaluate your performance at the all-India level, you can register for Mock Test Series provided by CollegeDekho and other various coaching centres. This will help you scale your performance and get a better estimate of where you stand.
Incorporate Class 11th Syllabus in your Preparation:

As JEE Main syllabuscomprises both class 11 and class 12 syllabus, it is essential to revise the topics that you have covered in class 11.
Start revising topics that you were good at as they will take less time and will allow you to prepare for board exams as well.
Devote at least two days of your week to prepare for topics that you studied in class 11. This way you will be able to cover the syllabus for both JEE Main and Class 12 Board exams.
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As long as you have focused on clearing your concepts, preparing for class 12 board exams and JEE Main 2020 will not be difficult. Keep calm and keep preparing.

All the best!