Hello Aspirant,

Hope you are doing well.

As you said you are in 12th standard so if you start your preparation for JEE Mains from now then it will help you very much in your future as less amount of pressure you faced after the broad exam and you have chances to score a very good rank in JEE Mains exam.

But it will not that much easy for you to manage both side if you manage your broad exam preparation as well as JEE Mains preparation then it will be very much appriciateable. Also you will get more time to prepare yourself in that subjects where you face difficulties.

Go through this link to get the syllabus of JEE Mains that will helps you in your preparation :

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

Hope this information helps you.

Best Wishes!