I have zero preparation of JEE mains and I've got around 80% marks in my MP Board class 12th. How can I get admission in a decent engineering college in MP?

Hello Siddhi,
It is very important to give as many tests as possible to improve your chances at gettin am admission in a reputed college in Madhya Pradesh. You have a good score in your boards.
You can still try and prepare for JEE Mains, although the time left now is hardly is sufficient for you to cover all the topics. You can choose to focus on the topics that you are familiar with and have a good understanding on. Also do not skip the easy topics. Write a mock test everyday. This will surely benefit you by showing you your level of preparation for the examinations.
Along with JEE Mains, you can also take the MP PET(Madhya Pradesh Pre-Engineering Test) and score well to improve your chances of getting an admission into a good college. The MP PET score will allow you to gain access to apply for an admission into various engineering colleges in Madhya Pradesh. So it is highly imperative that you spend the remaining time focusing on these exams to realise your dream of getting into a good college.
Some of the best colleges for engineering in Madhya Pradesh are:
1. IIT Indore
2. MANIT Bhopal
3. SIRT(Sagar Institute of Research and Technology)
4. AITR (Acropolis Institute of Technology and Research)
5. Oriental Institute of Science and Technology
6. Institute of Engineering and Sciences

I hope this helps. Good luck.