Sir please tell me the good way to prepare for jee advanced because I am a 12th student and I did not study more for jee advanced

Abhishek IIT-JEE (JEE Advanced) is one of the toughest exams in the world. You can't do the preparation within few weeks for most of the students even years preparation is not sufficient to clear JEE exam.  But still, for the last day preparations, I will recommend you to revise the whole syllabus, concepts, basics in first 5 days and then in next 10 days you should give a few mock tests so as to know which type of questions can come and where you stand in terms of marks you will get and finally again in last 3-4 days revise all the theory part.

Sir please tell me one more thing, am I eligible for advanced on basis of marks obtained in jee mains because I have 20 in physics and 35 in mathematics and bad -5 in chemistry. Now please tell me.

Yes, you are still eligible as there is no individual subject cut-off in JEE Mains. There is only one total marks cut-off and with 50 marks in OBC category you are eligible as the cut-off is 45 marks.

Sir my CRL rank is 240922 and OBC NCL rank is 77908 and my jee main score is 50. Am I still eligible for jee advanced. Sir please tell me soon

Your JEE main score is 50 in the OBC category where eligibility cut-off is 45 marks. Yes, boss, you are eligible for giving JEE Advanced exam. 

You saying that 74 is qualifying marks. Then tell me right that I may satisfy

Yes