Hello Gobinath,

To talk about the syllabus of JEE Mains syllabus is no different from 12th standard syllabus it contains mostly all the topics of 11th and 12th but in a detailed way. You canb check the syllabus by going through this link,

https://images.careers360.mobi/sites/default/files/JEE-Main-2018-Syllabus.pdf

So yes if you have studies your 11 -12th syllabus thoughly I don't think you'll find much difficulty in JEE mains though you need to prepare separately for JEE Advance as it's level is much difficult you can also go through some key points of NCERT as there's much time left you can't read full chapters and don't forget to do some model papers for your practice.

Good Luck!!