Hello,
If you want a great score in jee advanced 2020, you should have definetly started your preparation. You need to be thorough with your ncert textbooks (all subjects) especially chemistry. You should definitely take as many mock tests with good standards as much as you can. Go through TMC publication books. They are amazing. Try solving all the questions in the books. The most important thing is, you have to go through Concepts of physics book. Solve the excercise problems. They give a lot of exposure to the kind of questions you need to answer for jee advanced. 
Good Luck!