Mainly, syllabus of jee mains is completely from ncert books.Prepare ncert books thoroughly.
Prepare Inorganic and Organic Chemistry thoroughly from Ncert and some aprt like solid state state etc from it. In short, just prepare physics, chemistry of 11th & 12th from ncert.
For mathematics, read chapters like statistics, mean,tautology etc from ncert.
You can get syllabus from jee mains official site and must solve previous year questions. You will definetly get benefits from this.
All the best