NCERT Books will help you to clear your basic concepts which will help you to cover all the syllabus. Here are some preparation tips given below you can prefer:

Make a plan to manage Time Well- Make a time table and devide your time in two parts first your study plan & secondly your daily routine works. Within stipulated time you should finish the syllabus. You must draw certain goals. Depending on the difficulty level of the chapter or the topic, You must decide for yourself in how many days you will be able to finish it. Before proceeding towards the new topic, you must ensure that you finish the previous one and Revise thoroughly.

Go Thorough with the Syllabus - You must be well versed in the syllabus of JEE Main 2019. You should know the topics from which the questions will be asked. This will help you to divide your time.

Important Books to study: You should go through well with NCERT class 12th books and also go through the study material that is provided by the coaching centre from where they have studied. The other books you can refer-
Books for Mathematics

Trigonometry by S. L. Loney

Higher Algebra by Hall & Knight

Maths XI & XII by NCERT

Co-ordinate Geometry by S. L. Loney

Problems in Calculus of One Variable by I.A. Maron

Books for Physics

Problems in General Physics by I.E. Irodov

Fundamentals of Physics by Halliday, Resnick & Walker

Concepts of Physics Vol I and II by H.C. Verma

Books for Chemistry

Organic Chemistry by Paula Bruice Yurkanis

Organic Chemistry by Morrison & Boyd

Inorganic Chemistry by J.D. Lee

Numerical Chemistry by P. Bahadur

Refer Previous year papers: Previous year papers will help you to know the level of the quetions and also the coverage of each subject.

Good Luck!

NCERT books are good for JEE MAINS. And you should start reading them from class 11 only, because that is the right time.

Since questions are from both 11 and 12 so start reading them at the earliest.

https://engineering.careers360.com/articles/jee-main-2019

Check the link to know about the type of weightage given to chapters and prepare accordingly. The weightage doesn't vary much.

Best of Luck