CAT

Yes, your score in 12th is good and enough. Score well in your graduation too. And perform well in CAT. At least above 95 percentile to get calls from IIM.

Sorry to say but you scored really low. With this rank, you'll get a BDS college, that too if you are lucky enough. 204 is too less. However, you may look for private colleges of Bangalore (COMEDK), Maharashtra (DMER), UP. But these colleges charge a fairly high amount of fee. Good luck.

You should have above 95 percentile in CAT to get a call from IIM otherwise there is no chance to get a call from IIM .
GOOD LUCK

Well, CAT is the exam that totally depends on your IQ. Don't have any concerns with the state board and CBSE board. Study hard for CAT and give other entrance exams too for better percentile.

https://www.shiksha.com/mba/exams/cat
Here you will get all information related to CAT. So, check out the link. Good luck.

There are various scholarships that are available post seeking admission into IIM (A). One needs to score a minimum of 98+ percentile in CAT to get an invite for a PI (personal interview). Class 10th & 11th scores are not relevant here. You need at least 50% marks at your graduation level exams.

You can also take GMAT and then apply for IIM's.

You should target mock exams, a lot of them. Try to take at least 1-2 mocks every week and once you are comfortable with the syllabus you may increase that number to 5-6.