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Hello Siddhartha,
You should focus on the basics of the subjects as for now. Follow the NCERT books and solve as many problems as you can. After completing class 12th, you need to appear for various entrance examinations. I would suggest, you should appear for IIT JEE Mains and Advanced exam. These two entrance examinations provide admissions in IITs/ NITs and other government colleges, which are cost-effective.
I would also suggest, you should appear for individual state examinations (OJEE in Odisha, EAMCET in Andhra Pradesh/ Telangana, COMEDK in Karnataka, etc.). These exams also guarantee you good colleges and serve as a backup in case things don't end up the way you've planned for IIT-JEE.
Every state has its own share of good engineering colleges. So, don't underestimate or overlook them. They can prove to be vital. The more the entrance examinations you appear for, the more the options you will have for choosing colleges.
Certain private colleges like SRM, KIIT - Odisha, GITAM - Vishakhapatnam and ITER - Odisha conduct their own entrance examinations. You can opt for these as well.
After the results of those entrance examinations are announced, if you have a valid score and rank, you can attend the counselling procedure for the respective entrance examination. During the counselling procedure, you can select a college of your choice and the branch of engineering.
I hope it helps. If so, kindly, upvote this answer. If you've any further questions, kindly revert.