For JEE Main, Firstly i would suggest you to go through well with NCERT books it will help you to build your concept.

Here are some preparation tips you can follow:

1. Know your Syllabus Well: Check on the full syllabus of JEE Main 2018 before laying out your schedule for the upcoming studies. you will be able to know which units and topics are to be studied under each subject.

2. Work on a Time Table:  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, you will be able to be well prepared in them for the upcoming examination.

5. Write Notes and Prepare Flash Cards: 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. Similarly, sample papers will let you practically apply all of the topics you have been studying till now. Preparation using mock test and sample papers will let you know how much syllabus you have covered and where you will need to work to improve.

For KCET it is only based on your class 11th & 12th Karnataka board so if you are well prepare with this then you can crack this easily.