Hello Pankaj,

1. You should be well aware of the fact that these exam are subject to a lot of internal and external factors. It might be possible that you deserved much more, but simply couldn’t perform well on that day. This doesn’t mean your chance is over. You have potential, you just couldn’t prove it this time. Rather take it as a challenge to improve next time.
2. So step one is to get motivated again. Talk to your teachers and parents, they will help. Many people perform poorly in JEE(Main) and do well in JEE(Advanced). Not an issue.
3. Then analyze your paper of JEE(Main) 2018 and see why exactly did you lose marks? Were you not aware of the concepts? Did you commit too many silly mistakes? Did you get panicked? Was your speed too slow? Write it down somewhere and analyze each and every question. You will get to your mistakes.
4. Make a list of all your weak topics, strong topics and average topics. I recommend making the average ones strong, because JEE(Advanced) needs strong concepts. Start practicing questions from coaching sheets. COMPLETE YOUR COACHING MATERIAL BEFORE ANYTHING ELSE (assuming you are from a good coaching). Try to cover easy topics first because you want to get into IIT XYZ, your aim should not be IITB CSE or something for now. First ensure that you get an IIT and then go for improving your rank by practicing tough questions.
5. Give all the tests of your coaching, and solve all the assignments given. Clear your doubts then and there.
6. Improve examination temperament because that is the most important thing. Give a lot of tests and apply point 3 above on each test.
7. Try a lot of questions and see solutions after you do it/if you can’t solve after sufficient time. Make sure you know each and every question you do by heart.
8. Physics and Mathematics will need huge amount of problem solving. Do it. Learn all important reactions in chemistry. It will save you on the exam day.
9. Give maximum time to studies. MAXIMUM.
10. Your study material should be your coaching material and past papers only. Try covering as much as possible.

Don't worry just cover your coaching material first and foremost.