I have 90 percent in 10th, 85 in 12th and 61 in BTech. Is there any chances of getting admission in an IIM? 

Definitely, with this score, there is a possibility of getting admission at an IIM. There are also certain eligibility criteria that you must fulfill to take admission in IIMs. 

Eligibility:

• CAT score will play a major role whether you get admission or not. So, first of all, make sure that you score well in the exam. 
• Concentrate on your studies and focus on the elements that are in your hand now like CAT score, WAT, and PI. If you perform well and score high in these sections, the rest of the path becomes easy. 
• Try to score the required cut off so that you can get your stream of choice.

Preparation Tips:

• Practice as many mock tests as possible. There are many online websites where you can find CAT mock tests. Practice well and make your problem-solving skills efficient. Mock tests will help you analyze your strengths and weaknesses. This way, you can manage time and work upon your shortcomings. If you think that you are lacking somewhere, give extra 2 hours per day to that portion.
• Students who are weak in Verbal Ability should read routinely. 
• For Quantitative Aptitude, go through secondary textbooks. 
• If you are not a pro at DI/LR, make sure to settle organized and unstructured riddles normally. 

Remember one thing, the quality of your answers will help you score in the desired area/s so exceed your expectations in the quality of answers. Improve your score by improving your exactness. This will also help in your curriculum.