Hello Sarika,

In order to score a good marks in the JEE Main exam, the following tips could be helpful-
Your basics in Physics, Chemistry & Mathematics should be very clear upto 12th std. You should read the NCERT books for basics. Then HC Verma for Physics, RD Sharma for Mathematics, OP Tendon for Physical & Inorganic Chemistry, and Organic Chemistry of "Arihant Publication" for detail or extra knowledge. 
You should also try to solve the previous years question papers, this helps too much and you should also give the mock test to rectify yourself and build your confidence level.
For preparation of exam, the most important thing is your confidence after that you should know the whole syllabus and make your plan properly for learning new chapters, solving the numericals and revision. All three things should be the part of your preparation strategy in daily routine.
The main thing is that you have to build your concentration level, then only you can stay focused on your goal. I would like to say that please limit the study time between 10 to 12 hours and please increase your concentration and speed of reading, understanding and solving the problems then you will not face any kind of difficulty.
For more preparation tips, importants topics & books, you can go through the link given below
Https://engineering.careers360.com/articles/jee-main-2019
In this way, you can prepare well for the JEE Main and the JEE Advanced.
Thanks for query on this platform, you may visit again for further queries.
All the best.

Hello.

You need to be thorough with the NCERT books, try making notes of important formulae and theorems. Most of the questions of the Jee are based on your NCERT books. Try to focus more on chemistry as it is usually easier than the other 2 subjects, try to maximize your score, avoid attempting the question if you don't have an idea of how to solve it as negative marking severely reduces your rank. Try to solve as many question papers as possible to gauge your speed and accuracy. You could also join the knockout course to get assistance in your preparations:

https://www.entrance360.com/knockout-jee-main-april/

Good luck!