Dear Aspirant, I would say, you have good marks in your 10th boards, which means your basics are well cleared. If you study the same way in your 12th boards as well, then your basic concepts on three of the subjects Physics, Chemistry and Maths, will already be very clear.

Now comes the answer for objectives for Jee Mains. Well, objectives are nothing but multiple choice questions on your core concepts. Practice is the key. You can revise the chapter, in which you are feeling weak. Following proper books and a good amount of practice of MCQs is the only way to success here.

For some last minute suggestions you can check the website

https://engineering.careers360.com/articles/jee-main-preparation-tips

You can also check out some strategy before exams

https://learn.careers360.com/engineering/jee-main/

Practice more and more, and you would soon find, you are no more weak in objectives.

I wish you all the best for your exams.