NIT

First of all, congratulations to get good marks in your 12th. Taking admission in NIT or IIIT is dream of many students in India. To get admission in NIT or IIIT, you need to appear for JEE Mains exam conducted by NTA every year. Every year 7-8lakh students appear to give this exam
There are 31 NIT and 25 iiit in india in which there are approximately 25,000 Sets for B.Tech in all NIT and approximately 5000 seats for B.Tech in all iiit. As you got 89 percentile in JEE Mains score, to get admission in NIT or IIIT bassd on your this score, it depends on that from which category you belong like general, OBC, SC, ST, PwD
If you are from general category, then you will not get any NIT and iiit
But if you are from SC or ST or PwD category, then you will surely get any NIT or iiit with best B.Tech branch as per your choice.

Getting admission in NIT or IIIT is not so easy, it is dream of many students in India, they start preparation from class 11 to get it, even many students prepare after 12th to get admission in NIT or IIIT
The admission process of NITs is done through JEE Main counselling conducted by Joint Seat Allocation Authority (JoSAA). Candidates shortlisted for the counselling process on the basis of merit list are required to register online in order to participate in the process. To get admission in it, you would have to start preparing for JEE Mains as soon as possible, and have to score a very good rank in it then register for jossa counselling. If you are from General category, then you have to achieve very good rank to get mechanical engineering it top NIT and IIIT, if you are from SC ST category, then it is not necessary to get god rank in JEE Mains, you can get it even if your rank is under 1,50,000.

JEE Mains score is used by various college for students to pursue B.Tech in different branches. Different types of colleges have their own criteria for admission
NIT IIIT recruit only on basis of JEE Mains, Central University government University takes admission based on JEE Mains or CUET
As your JEE Mains rank in ST category is under 20,000, you will allocated best NIT or iiit college, also you may allocated B.Tech in CSE branch from top NIT colleges
different NIT and iiit are best in different B.Tech branches, you should choose them wisely according to your desired branch. Here are top NIT and iiit college list
NIT DELHI
NIT CALICUT
NIT JAIPUR
NIT KURUKSHETRA
NIT TRICHY
NIT BHOPAL
NIT ALLAHABAD
IIIT HYDERABAD
IIIT ALLAHABAD
IIIT BHUVNESHWAR
IIIT LUCKNOW
IIIT MANIPUR
IIIT KARNATAK.