Hello,

The best way to start preparing is by knowing the syllabus and exam pattern.For JEE Mains there will be a total of 75 questions ( 25 questions each from Physics, Chemistry and Mathematics).You should give importance in understanding the concepts.Try solving maximum  numerical problems.Also solve previous question papers and mock tests .You can start with the subject of your choice but keep in mind that all the three subjects have equal weight age.

You can also make use of the link below to know more about the important topics and syllabus:

https://engineering.careers360.com/articles/jee-main-syllabus-weightage