This time is sufficient to get any good  rank in JEE Mains or Advanced. I would suggest you to go through well with NCERT books as you told already that you are in class 11th then you can do the preparation well for the exam as the syllabus of the exams is from claas 11th & 12th mostly so if you are clear with the basics, You can easily cover whole the syllabus. Here are some preparation tips you can follow:

1. Know your Syllabus Well: Before starting on the preparation process, there is one important step to be completed. Check on the full syllabus of JEE Main 2018 before laying out your schedule for the upcoming studies.

2. Work on a Time Table: After carefully checking the syllabus of JEE Main work towards creating a perfect timetable which will fit your schedule while leaving enough time to prepare for the examination within the days left. Make sure to add short and sweet breaks in between for some relaxation.

3. Make your Basic Concepts Strong: While it is essential to study the topics mentioned in the official syllabus, it is also advised to study the basic concepts well first. By knowing the basic concepts of the units involved in the examination, you will be able to understand each topics well and in depth. This will result in a greater understanding of the topics and will make it simpler for you to study complicated concepts present in the syllabus.

4. Improve your Weak Points: Stressed over some topics or formulas which you just cannot seem to understand? Give more time and focus on these topics to improve it. Keep small notes and constantly revise those concepts which has been hard to understand. By constantly studying the topics you find difficult.

5. Write Notes and Prepare Flash Cards: Like discussed in the previous strategy, keeping notes greatly helps during the preparation process. Notes written in small diaries are highly accessible and you will be able to study from them anywhere anytime. By writing down those complicated theories and formula and revising them, you will be able to remember and understand them more efficiently. Similarly, create flash cards to help you with quick studies and revision.

6. Attempt Mock Test and Solve Sample Papers: Mock tests will give you an almost real experience of how the examination will be. By solving mock tests, you will be able to work on time management while at the same time understanding the question paper pattern.

Revision: While studying it is important to revise those topics which has already been studied. It is advised that each day, a certain amount of time should be divided for revision purposes. Revision is very important as the syllabus is quite vast and all topics have similar importance level which has to be remembered and known by you.