Hey Rini,

The best way to reduce careless mistake is to practice more and more questions and learn from the mistakes you are commiting.In addition to that don't panic and have faith and be confident.

All the best for your Exam!

Thanks

You should note your mistakes. When you will give mock exams than you should analyze your mistakes . You should practice lots of questions. Try to analyze your strong area and first attempt that area. Try to analyze time give specific time for all subjects. Don't waste more than 2 min on 1 question. Don't do guess estimate. Answer those questions for which you are sure about . Solve questions and fill circle on your answer sheet simultaneously. Bcz sometimes in hurry you can do mistake and 1 wrong circle filling can incorrect your all other ans. Do as much mock test as you can.You should remember all the formula and short tricks to Solve ans.Dont give stress to your mind. Cool your mind before the exam. Don't get nervous in the exam . 

The best way to avoid any mistake in JEE MAINS, is to practice alot specially previous year questions papers, dont get nervours, solve the questions in minimum time.