How to start preparing neet exam without any coaching classes from class 12?

I would highly recommend coaching, it is an indispensible part of NEET preparation. If possible, look into taking coaching from a reputable coaching institute with years of experience, it will increase your chances of getting a good NEET score. If you do not want to attend coaching classes, here are the tips I would suggest:

1) Self-study - While coaching is extremely valuable, self-study is the key to success. Create a study timetable that suits your habits. Try to solve as many questions as possible after studying a certain topic. It's easier to complete the more time-consuming or important topics first, and then leave the easier or less important topics for later. Don't leave any topics. Know the NEET syllabus, and focus on important topics. Here are the most important topics for NEET, make sure you study these very well:

https://medicine.careers360.com/articles/neet-chapter-wise-weightage-and-important-topics

2) Regular revision - Set time aside to revise whatever you have studied. Learning without revision is futile. Revision reinforces your understanding, sharpens memory and clears concepts thus making it easier to get more questions right.

3) Mock exams - Practice tests and mock exams give an idea of where you stand in terms of your preparation, what your strengths and weaknesses are. It also betters your speed and hones time management skills. Repeatedly doing mock exams will accustom you to the rigor and pressure of the actual exam, thereby increasing your confidence and ease in the real exam. Also solve plenty of past NEET papers, this is very important.

4) Use the right materials - Do not buy too many books or use too many resources, use the ones that you find the easiest to understand (especially the NCERT textbooks).

5) Don't use guesswork - Guesswork is more risky than leaving the question since there is negative marking. Attempt the questions you're sure of. You will be able to attempt more questions correctly with a lot of practice and effective studying.

6) Take time off - It is important to balance your studying with breaks so that you do not burn out. Indulge in a hobby to keep you relaxed and refreshed to start another study session. Know your distractions and avoid them at all costs. They will waste time, which is scarce at this point. Get adequate sleep every night. Don't sleep during your study time and don't study during your sleep time.

7) Passion, not pressure - Be passionate and motivated towards your studies. Do not get overwhelmed, stay focused on your dream and align your schedule to meet your goal. Never be overconfident, neither should you feel unmotivated or under too much pressure to function. Eat well, sleep well and study well. Most importantly, never forget your dream even in the most difficult moments!

Good Luck!