Hello student you can prepare for both but I'll recommend you to first focus on Jee main as its the basic without it you can't easily prepare for jee advance.

Prefer NCERT to build your basic concept and make sure you do each and every example and exercise questions of Ncert before jumping to a reference book.

After completing ncert thoroughly do some reference book.
You can also subscribe careers360 jee knockout package which will help you in preparation.
https://learn.careers360.com/knockout-jee-main-april/

Eligibility - candidates must passed or appearing class 12 with physics chemistry and mathematics with minimum 75 percent marks aggregate from a recognized board

Feel free to comment if you have any doubt
Good luck

Dear Aspirant,

If you are have physics, chemistry, mathematics, english and one subject in your 12th and able to score 75% in your final exams of boards then you are eligible for JEE main and advanced.

Moreover, for advance you can appear only in the year of passing and the immediately next year after that.

So if you comply with this eligibility, then you can start to prepare.

Here are few links for you:-

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria

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